Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

The official, unofficial slogan of Perl 6 is "Second System Syndrome Done
Right!". After you read this Apocalypse you will at least be certain
that we got the "Second System" part down pat. But we've also put in
a little bit of work on the "Done Right" part, which we hope you'll
recognize. The management of complexity is complex, but only if you
think about it. The goal of Perl 6 is to discourage you from thinking
about it unnecessarily.

Speaking of thinking unnecessarily, please don't think that everything
we write here is absolutely true. We expect some things to change as
people point out various difficulties. That's the way all the other
Apocalypses have worked, so why should this one be different?

When I say "we", I don't just mean "me". I mean everyone who has participated
in the design, including the Perl 6 cabal, er, design team, the readers
(and writers) of the perl6-language mailing list, and all the participants
who wrote or commented on the original RFCs. For this Apocalypse we've
directly considered the following RFCs:

These RFCs contain many interesting ideas, and many more "cries for help".
Usually in these Apocalypses, I discuss the design with respect to each
of the RFCs. However, in this case I won't, because most of these RFCs
fail in exactly the same way--they assume the Perl 6 object model to
be a set of extensions to the Perl 5 object model. But as it turns out,
that would have been a great way to end up with Second System Syndrome
Done Wrong. Perl 5's OO system is a great workbench, but it has some
issues that have to be dealt with systematically rather than piecemeal.

It has often been claimed that Perl 5 OO was "bolted on", but that's
inaccurate. It was "bolted through", at right angles to all other reference
types, such that any reference could be blessed into being an object.
That's way cool, but it's often a little too cool.

It's too hard to treat built-in types as objects when you want to. Perl
5's tie interface helps, but is suboptimal in several ways,
not the least of which is that it only works on variables, not values.

Because of the ability to turn (almost) anything into an object, a derived
class had to be aware of the internal data type of its base class. Even
after convention settled on hashes as the appropriate default data structure,
one had to be careful not to stomp on the attributes of one's base class.

Some people will be surprised to hear it, but Perl is a minimalist language
at heart. It's just minimalistic about weird things compared to your
average language. Just as the binding of parameters to @_
was a minimalistic approach, so too the entire Perl 5 object system
was an attempt to see how far you could drive a few features. But many
of the following difficulties stem from that.

In Perl 5, a class is just a package, a method is just a subroutine,
and an object is just a blessed referent. That's all well and good,
and it is still fundamentally true in Perl 6. However, Perl 5 made the
mistake of reusing the same keywords to express similar ideas. That's
not how natural languages work--we often use different words to express
similar ideas, the better to make subtle distinctions.

Because Perl 5 reused keywords and treated parameter binding as something
you do via a list assignment at runtime, it was next to impossible
for the compiler to tell which subroutines were methods and which ones
were really just subroutines. Because hashes are mutable, it was difficult
to tell at compile time what the attribute names were going to be.

The Perl 5 solution to the previous problem was to declare more things
at compile time. Unfortunately, since the main way to do things at compile
time was to invoke use, all the compile-time interfaces
were shoehorned into use's syntax, which, powerful though
it may be, is often completely inside-out from a reasonable interface.
For instance, overloading is done by passing a list of pairs to use,
when it would be much more natural to simply declare appropriate methods
with appropriate names and traits. The base and fields
pragmas are also kludges.

Because of the flexibility of the Perl 5 approach, there was never any
"obvious" way to do it. So best practices had to be developed by each
group, and of course, everyone came up with a slightly different solution.
Now, we're not going to be like some folks and confuse "obvious" with
"the only way to do it". This is still Perl, after all, and the flexibility
will still be there if you need it. But by convention, there needs to
be a standard look to objects and classes so that they can interoperate.
There's more than one way to do it, but one of those is the standard
way.

One of the big advances in Perl 5 was that a program could be in charge
of its own compilation via use statements and BEGIN
blocks. A Perl program isn't a passive thing that a compiler has its
way with, willy-nilly. It's an active thing that negotiates with the
compiler for a set of semantics. In Perl 6 we're not shying away from
that, but taking it further, and at the same time hiding it in a more
declarative style. So you need to be aware that, although many of the
things we'll be talking about here look like declarations,
they trigger Perl code that runs during compilation. Of such methods
are metaclasses made. (While these methods are often triggered by grammar
rule reductions, remember from Apocalypse 5 that all these grammar rules
are also running under the user's control. You can tweak the language
without the crude ax of source filtering.)

In looking for an "obvious" way to conventionalize Perl's object system,
we shouldn't overlook the fact that there's more than one obvious way,
and different approaches work better in different circumstances. Inheritance
is one way (and typically the most overused), but we also need good
support for composition, delegation, and parametric types. Cutting across
those techniques are issues of interface, implementation, and mixtures
of interface and implementation. There are multiple strategies for ambiguity
resolution as well, and no single strategy is always right. (Unless
the boss says so.)

So to summarize this summary, what we're proposing to develop is a set
of conventions for how object orientation ought to work in Perl 6--by
default. But there should also be enough hooks to customize things to
your heart's content, hopefully without undue impact on the sensibilities
of others.

And in particular, there's enough flexibility in the new approach that,
if you want to, you can still program in a way much like the old Perl
5 approach. There's still a bless method, and you can still
pretend that an object is a hash--though it isn't anymore.

However, as with all the rest of the design of Perl 6, the overriding
concern has been that the language scale well. That means Perl has to
scale down as well as up. Perl has to work well both as a first language
and as a last language. We believe our design fulfills this goal--though,
of course, only time will tell.

One other note: if you haven't read the previous Apocalypses and Exegeses,
a lot of this is going to be complete gobbledygook to you. (Of course,
even if you have read them, this might still be gobbledygook.
You take your chances in life...)

Before we start talking about all the hard things that should be possible,
let's look at an example of some of the easy things that should be easy.
Suppose we define a Point object that (for some strange reason) allows
you to adjust the y-axis but not the x-axis.

class Point {
has $.x;
has $.y is rw;

method clear () { $.x = 0; $.y = 0; }
}

my $point = Point.new(x => 2, y => 3);

$a = $point.y; # okay
$point.y = 42; # okay

$b = $point.x; # okay
$point.x = -1; # illegal, default is read-only

$point.clear; # reset to 0,0

If you compare that to how it would have to be written in Perl 5, you'll
note a number of differences:

It uses the keywords class and method
rather than package and sub.

The attributes are named in explicit declarations rather than implicit
hash keys.

It is impossible to confuse the attribute variables with ordinary
variables because of the extra dot (which also associates the attributes
visually with method calls).

Perhaps most importantly, we did not have to commit to using a
hash (or any other external data structure) for the object's values.

We didn't have to write a constructor.

The implicit constructor automatically knows how to map named arguments
to the attribute names.

We didn't have to write the accessor methods.

The accessors are by default read-only outside the class, and you
can't get at the attributes from outside the class without an accessor.
(Inside the class you can use the attributes directly.)

The invocant of the clear method is implicit.

And perhaps most obviously, Perl 6 uses . instead
of -> to dereference an object.

Now suppose we want to derive from Point, and add a z-axis. That's just:

The implicit constructor automatically sorts out the named arguments
to the correct initializers for you. If you omit the z value, it will
default to 123. And the new clear method calls the old
clear method merely by invoking next, without the dodgy
"super" semantics that break down under MI. We also declared the $:z
attribute to be completely private by using a colon instead of a dot.
No accessor for it is visible outside the class. (And yes, OO purists,
our other attributes should probably have been private in the first
place...that's why we're making it just as easy to write a private attribute
as a public one.)

If any of that makes your head spin, I'm sure the following will clear
it right up. :-)

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

A class is what provides a name and a place for the abstract behavior
of a set of objects said to belong to the class.

As in Perl 5, a class is still "just a funny package", structurally speaking.
Syntactically, however, a class is now distinct from a package or a
module. And the body of a class definition now runs in the context of
a metaclass, which is just a way of saying that it has a metaclass instance
as its (undeclared) invocant. (An "invocant" is what we call the object
or class on behalf of which a method is being called.) Hence class definitions,
though apparently declarative, are also executing code to build the
class definition, and the various declarations within the class are
also running bits of code. By convention classes will use a standard
metaclass, but that's just convention. (A very strong convention, we
hope.)

The primary role of a class is to manage instances, that is, objects.
So a class must worry about object creation and destruction, and everything
that happens in between. Classes have a secondary role as units of software
reuse, in that they can be inherited from or delegated to. However,
because this is a secondary role, and because of weaknesses in models
of inheritance, composition, and delegation, Perl 6 will split out the
notion of software reuse into a separate class-like entity called a
"role". Roles are an abstraction mechanism for use by classes that don't
care about the secondary aspects of software reuse, or that (looking
at it the other way) care so much about it that they want to encapsulate
any decisions about implementation, composition, delegation, and maybe
even inheritance. Sounds fancy, but just think of them as includes of
partial classes, with some safety checks. Roles don't manage objects.
They manage interfaces and other abstract behavior (like default implementations),
and they help classes manage objects. As such, a role may only be composed
into a class or into another role, never inherited from or delegated
to. That's what classes are for.

Classes are arranged in an inheritance hierarchy by their "isa" relationships.
Perl 6 supports multiple inheritance, but makes it easy to program in
a single-inheritance style, insofar as roles make it easy to mix in
(or delegate, or parameterize) private implementation details that don't
belong in the public inheritance tree.

In those cases where MI is used, there can be ambiguities in the pecking
order of classes in different branches. Perl 6 will have a canonical
way to disambiguate these, but by design the dispatch policy is separable
from inheritance, so that you can change the rules for a given set of
classes. (Certainly the rules can change when we call into another language's
class hierarchy, for instance.)

Where possible, class names are treated polymorphically, just as method
names are. This powerful feature makes it possible to inherit systems
of classes in parallel. (These classes might be inner classes, or they
might be inner aliases to outer classes.) By making the class names
"virtual", the base classes can refer to the appropriate derived classes
without knowing their full name. That sounds complicated, but it just
means that if you do the normal thing, Perl will call the right class
instead of the one you thought it was going to call. :-)

(As in C++ culture, we use the term "virtual" to denote a method that
dispatched based on the actual runtime type of the object rather than
the declared type of the variable. C++ classes have to declare their
methods to be virtual explicitly. All of Perl's public methods are virtual
implicitly.)

You may derive from any built-in class. For high-level object classes
such as Int or Num there are no restrictions
on how you derive. For low-level representational classes like int
or num, you may not change the representation of the value;
you may only add behaviors. (If you want to change the representation,
you should probably be using composition instead of inheritance. Or
define your own low-level type.) Apart from this, you don't need to
worry about the difference between int and Int,
or num and Num, since Perl 6 will do autoboxing.

An incomplete class definition makes use of the ... ("yada,
yada, yada") operator:

class Dog is Mammal {...}

The declaration of a class name introduces the name as a valid bare identifier
or name. In the absence of such a declaration, the name of a class in
an expression must be introduced with the :: class sigil,
or it will be considered a bareword and rejected, since Perl 6 doesn't
allow barewords. Once the name is declared however, it may be used as
an ordinary term in an expression. Unlike in Perl 5, you should not
view it as a bareword string. Rather, you should view it as a parameterless
subroutine that returns a class object, which conveniently stringifies
to the name of the class for Perl 5 compatibility. But when you say

Dog.new()

the invocant of new is an object of type Class,
not a string as in Perl 5.

Unmodified, a class declaration always declares a global name. But if
you prefix it with our, you're defining an inner class:

class Cell {
our class Golgi {...}
...
}

The full name of the inner class is Cell::Golgi, and that
name can be used outside of Cell, since Golgi
is declared in the Cell package. (Classes may be declared
private, however. More later.)

A class declaration may apply various traits to the class. (A trait is
a property applied at compile time.) When you apply a trait, you're
accepting whatever it is that that trait does to your class, which could
be pretty much anything. Traits do things to classes. Do not
confuse traits with roles, which are sworn to play a subservient role
to the class. Traits can do whatever they jolly well please to your
class's metadata.

Now, the usual thing to do to a class's metadata is to insert another
class into its ISA metadata. So we use trait notation to install a superclass:

class Dog is Mammal {...}

To specify multiple inheritance, just add another trait:

class Dog is Mammal is Pet {...}

But often you'll want a role instead, specified with does:

class Dog is Mammal does Pet {...}

More on that later. But remember that traits are evil. You can have traits
like:

class Moose is Mammal is stuffed is really(Hatrack) is spy(Russian) {...}

So what if you actually want to derive from stuffed? That's
a good question, which we will answer later. (The short answer is, you
don't.)

Now as it happens, you can also use is from within the class.
You can also put the does inside to include various roles:

class Dog {
is Mammal;
does Pet;
does Servant;
does Best::Friend[Man];
does Drool;
...
}

In fact, there's no particular reason to put any of these outside the
braces except to make them more obvious to the casual reader. If we
take the view that inheritance is just one form of implementation, then
a simple

class Dog {...}

is sufficient to establish that there's a Dog class defined
out there somewhere. We shouldn't really care about the implementation
of Dog, only its interface--which is usually pretty slobbery.

That being said, you can know more about the interface at compile time
once you know the inheritance, so it's good to have pulled in a definition
of the class as well as a declaration. Since this is typically done
with use, the inheritance tree is generally available even
if you don't mark your class declaration externally with the inheritance.
(But in any event, the actual inheritance tree doesn't have to be available
till runtime, since that's when methods are dispatched. (Though as
is often the case, certain optimizations work better when you give them
more data earlier...))

A class is used directly by calling class methods, and indirectly by
calling methods of an object of that class (or of a derived class that
doesn't override the methods in question).

Classes may also be used as objects in their own right, as instances
of a metaclass, the class MetaClass by default. When you
declare class Dog, you're actually calling a metaclass
class method that constructs a metaclass instance (i.e., the Dog
class) and then calls the associated closure (i.e., the body of the class)
as a method on the instance. (With a little grammatical magic thrown
in so that Dog isn't considered a bareword.)

The class Dog is an instance of the class MetaClass,
but it's also an instance of the type Class when you're
thinking of it as a dispatcher. That is, a class object is really allomorphic.
If you treat one as an instance of Class, it behaves as if it were the
user's view of the class, and the user thinks the class is there only
to dispatch to the user's own class and instance methods. If, however,
you treat the object as an instance of MetaClass, you get
access to all its metaclass methods rather than the user-defined methods.
Another way to look at it is that the metaclass object is a separate
object that manages the class object. In any event, you can get from
the ordinary class object to its corresponding metaclass object via
the .meta method, which every object supports.

By the way, a Class is a Module, which in turn
is a Package, which in turn is an Object. Or
something like that. So a class can always be used as if it were a mere
module or package. But modules and packages don't have a .dispatch
method...

By default, classes in Perl are left open. That is, you can add more
methods later. (However, an application may close them.) For discussion
of this, see the section on "Open vs. Closed Classes".

Unlike in Perl 5, when you mention a package name in Perl 6 it doesn't
always mean a global name, since Perl 6 knows about inner classes and
lexically scoped packages and such. As with other entities in Perl such
as variables and methods, a scan is made for who thinks they have the
best definition of the name, going out from lexical scopes to package
scope to global scope in the case of static class names, and via method
inheritance rules in the case of virtual class names.

Note that ::MyClass and MyClass mean the same
thing. In Perl 6, an initial :: is merely an optional sigil
for when the name of the package would be misconstrued as something
else. It specifically does not mean (as it does in Perl 5) that it is
a top-level package. To refer to the top-level package, you would need
to say something like ::*MyClass (or just *MyClass
in places where the * unary operator would not be expected.)
But also note that the * package in Perl is not the "main"
package in the Perl 5 sense.

Likewise, the presence of :: within a package name like
Fish::Carp does not make it a global package name necessarily.
Again, it scans out through various scopes, and only if no local scopes
define package Fish::Carp do you get the global definition.
And again, you can force it by saying ::*Fish::Carp. (Or
just *Fish::Carp in places where the * unary
operator is not expected.)

You can interpolate a parenthesized expression within a package name
after any ::. So, these are all legal package names (or
module names, or class names):

Note in the last example that the final element of @multilevel
is taken to be the variable name. This may be illegal under use
strict refs, since it amounts to a symbolic reference. (Not that
the others aren't symbolic, but the rules may be looser for package
names than for variable names, depending on how strict our strictures
get.)

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

It is completely ignored outside of the current class. Since the name
is useful only in the current package, it makes no sense to try to qualify
it with a package name. While it's an inner class of sorts, it does
not override any class name from any other class because it lives in
its own namespace (a subnamespace of the current package), and there's
no way to tell if the class you're deriving from declares its own private
class of the same name (apart from digging through the reflection interfaces).

The colon is orthogonal to the scoping. What's actually going on in this
example is that the name is stored in the package with the leading colon,
because the colon is part of the name. But if you declared "my
class :Golgi" the private name would go into the lexical namespace
with the colon. The colon functions a bit like a "private" trait, but
isn't really a trait. Wherever you might use a private name, the colon
in the name effectively creates a private subspace of names, just as
if you'd prefixed it with "_" in the good old days.

But if were only that, it would just be encapsulation by convention.
We're trying to do a little better than that. So the language needs
to actively prevent people from accessing that private subspace from
outside the class. You might think that that's going to slow down all
the dispatchers, but probably not. The ordinary dispatch of Class.method
and $obj.method don't have to worry about it, because they
use bare identifiers. It's only when people start doing ::($class)
or $obj.$method that we have to trap illegal references
to colonic names.

Even though the initial colon isn't really a trait, if you interrogate
the ".private" property of the class, it will return true.
You don't have to parse the name to get that info.

We'll make more of this when we talk about private methods and attributes.
Speaking of methods...

Methods are the actions that a class knows how to invoke on behalf of
an object of that type (or on behalf of itself, as a class object).
But you knew that already.

As in Perl 5, a method is still "just a funny subroutine", but in Perl
6 we use a different keyword to declare it, both because it's better
documentation, and because it captures the metadata for the class at
compile time. Ordinary methods may be declared only within the scope
of a class definition. (Multimethods are exempt from this restriction,
however.)

To declare a method, use the method keyword just as you
would use sub for an ordinary subroutine. The declaration
is otherwise almost identical:

method doit ($a, $b, $c) { ... }

The one other difference is that a method has an invocant on
behalf of which the method is called. In the declaration above, that
invocant is implicit. (It is implicitly typed to be the same as the
current surrounding class definition.) You may, however, explicitly
declare the invocant as the first argument. The declaration knows you're
doing that because you put a colon between the invocant and the rest
of the arguments:

method doit ($self: $a, $b, $c) { ... }

In this case, we didn't specify the type of $self, so it's
an untyped variable. To make the exact equivalent of the implicit declaration,
put the current class:

method doit (MyClass $self: $a, $b, $c) { ... }

or more generically using the ::_ "current class" pronoun:

method doit (::_ $self: $a, $b, $c) { ... }

In any case, the method sets the current invocant as the topic, which
is also known as the $_ variable. However, the topic can
change depending on the code inside the method. So you might want to
declare an explicit invocant when the meaning of $_ might
change. (For further discussion of topics see Apocalypse 4. For a small
writeup on sub signatures see Apocalypse 6.)

A private method is declared with a colon on the front:

method :think (Brain $self: $thought)

Private methods are callable only by the class itself, and by trusted
"friends". More about that when we talk about attributes.

(Parens are always required around the argument list when a method call
with arguments is interpolated into a string.)

The parser will make use of whitespace at this point to decide
some things. For instance

$obj.method + 1

is obviously a method with no arguments, while

$obj.method +1

is obviously a method with an argument. However, the dwimmery only goes
as far as the typical person's visual intuition. Any construct too ambiguous
is simply rejected. So

$obj.method+1

produces a parse error.

In particular, curlies, brackets, or parens would be interpreted as postfix
subscripts or argument lists if you leave out the space. In other words,
Perl 6 distinguishes:

$obj.method ($x + $y) + $z # means $obj.method(($x + $y) + $z)

from

$obj.method($x + $y) + $z # means ($obj.method($x + $y)) + $z

Yes, this is different from Perl 5. And yes, I know certain people hate
it. They can write their own grammar.

While it's always possible to disambiguate with parentheses, sometimes
that is just too unsightly. Many methods want to be parsed as if they
were list operators. So as an alternative to parenthesizing the entire
argument list, you can disambiguate by putting a colon between the method
call and the argument list:

In order to talk to the scalar container itself, use the tied()
pseudo-function as you would in Perl 5:

if tied($scalar).constant {...}

(You may recall, however, that in Perl 6 it's illegal to tie any variable
without first declaring it as tie-able, or (preferably) tying it directly
in the variable's declaration. Otherwise the optimizer would have to
assume that every variable has semantics that are unknowable in advance,
and we would have to call it a pessimizer rather than an optimizer.)

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

The other form of method call is known as the "indirect object" syntax,
although it differs from Perl 5's syntax in that a colon is required
between the indirect object (the invocant) and its arguments:

doit $object: "a", "b", "c"

The colon may be omitted if there are no arguments (besides the invocant):

twiddle $thumb;
$x = new X;

Note that indirect object calls may not be directly interpolated into
a string, since they don't start with a sigil. You can always use the
$() expression interpolater though:

say "$(greet $lang), world!";

As in Perl 5, the indirect object syntax is valid only if you haven't
declared a subroutine locally that overrides the method lookup. That
was a bit of a problem in Perl 5 since, if there happened to be a new
constructor in your class, it would call that instead dispatching to
the class you wanted it to. That's much less of a problem in Perl 6,
however, because Perl 6 cannot confuse a method declaration with a subroutine
declaration. (Which is yet another reason for giving methods their own
keyword.)

Another factor that makes indirect objects work better in Perl 6 is that
the class name in "new X" is a predeclared object, not
a bare identifier. (Perl 5 just had to guess when it saw two bare identifiers
in a row that you were trying to call a class method.)

The indirect object syntax may not be used with a variable for the methodname.
You must use dot notation for that.

Because of precedence, the indirect object notation may not be used as
an lvalue unless you parenthesize it:

(mystate $object) = 42;
(findtail Dog: "fido") = Wagging::on;

You may parenthesize an argumentless indirect object method to make it
look like a function:

mystate($object) = 42;
twiddle($thumb);

The dispatch rules for methods and global multi subs conspire to keep
these unambiguous, so the user really doesn't have to worry about whether

close($handle);

is implemented as a global multi sub or a method on a $handle
object. In essence, the multimethod dispatching rules degenerate to
ordinary method dispatch when there are no extra arguments to consider
(and sometimes even when there are arguments). This is particularly
important because Perl uses these rules to tell the difference between

print "Howdy, world!\n"; # global multi sub

and

print $*OUT; # ordinary filehandle method

However, you must still put the colon after the invocant if there are
other arguments. The colon tells the parser whether to look for the
arguments inside:

doit($object: "a", "b", "c")

or outside:

doit($object): "a", "b", "c"

If you do say

doit($object, "a", "b", "c")

the first comma forces it to be interpreted as a sub call rather than
a method call.

(We could have decided to say that whenever Perl can't find a doit()
sub definition at runtime, it should assume you meant the entire parenthesized
list to be the indirect object, which, since it's in scalar context
would automatically generate a list reference and call [$object,"a","b","c"].doit(),
which is unlikely to be what you mean, or even work. (Unless, of course,
that's how you really meant it to work.) But I think it's much more
straightforward to simply disallow comma lists at the top level of an
indirect object. The old "if it looks like a function" rule applies
here. Oddly, though, function syntax is how you call multisubs in Perl
6. And as it happens, the way the multisub/multimethod dispatch rules
are defined, it could still end up calling $object.doit("a",
"b", "c") if that is deemed to be the best
choice among all the candidates. But syntactically, it's not an indirect
object. More on dispatch rule interactions later.)

The comma still doesn't work if you go the other way and leave out the
parens entirely, since

doit $object, "a", "b", "c";

would always (in the absence of a prior sub declaration) be parsed as

(doit $object:), "a", "b", "c";

So a print with both an indirect object and arguments has to look like
one of these:

You'd only need the parens if the invocant expression contained operators
lower in precedence than comma (comma itself not being allowed). Basically,
if it looks confusing to you, you can expect it to look confusing to
the compiler, and to make the compiler look confused. But it's a feature
for the compiler to look confused when it actually is confused.
(In Perl 5 this was not always so.)

Note that the disambiguating colon associates with the closest method
call, whether direct or indirect. So

print $obj.meth: "Howdy, world!\n";

passes "Howdy, world!\n" to $obj.meth rather
than to print. That's a case where you ought to have parenthesized
the indirect object for clarity anyway:

A private method does not participate in normal method dispatch. It is
not listed in the class's public methods. The .can method
does not see it. Calling it via normal dispatch raises a "no such method"
exception. It is, in essence, invisible to the outside world. It does
not hide a base class's method of the same name--even in the current
class! It's fair to ask for warnings about name collisions, of course.
But we're not following the C++ approach of making private methods visible
but uncallable, because that would violate encapsulation, and in particular,
Liskov
substitutability. Instead, we separate the namespaces completely
by distinguishing the public dot operator from the private dot-colon
operator. That is:

The inclusion of the colon prevents any kind of "virtual" behavior. Calling
a private method is illegal except under two very specific conditions.
You can call a private method :think on an object $brain
only if:

The class of $brain is explicitly declared, and the
declared class is either the class definition that we are in or
a class that has explicitly granted trust to our current class,
and the declared class contains a private :think method.
Or...

The class of the $brain is not declared, and the current
class contains a private :think method.

The upshot of these rules is that a private method call is essentially
a subroutine call with a method-like syntax. But the private method
we're going to call can be determined at compile time, just like a subroutine.

Class methods are called on the class as a whole rather than on any particular
instance object of the class. They are distinguished from ordinary methods
only by the declared type of the invocant. Since an implicit invocant
would be typed as an object of the class and not as the class itself,
the invocant declaration is not optional in a class method
declaration if you wish to specify the type of the invocant. (Untyped
explicit invocants are allowed to "squint", however.)

Some methods are intended to be inherited by derived classes. Others
are intended to be reimplemented in every class, or in every class that
doesn't want the default method. We call these "submethods", because
they work a little like subs, and a little like methods. (You can also
read the "sub" with the meaning it has in words like "subhuman".)

Typically these are (sub)methods related to the details of construction
and destruction of the object. So when you call a constructor, for instance,
it ends up calling the BUILDALL initialization routine
for the class, which ends up calling the BUILD submethod:

submethod BUILD ($a, $b, $c) {
$.a = $a;
$.b = $b;
$.c = $c;
}

Since the submethod is doing things that make sense only in the context
of the current class (such as initializing attributes), it makes no
sense for BUILD to be inherited. Likewise DESTROY
is also a submethod.

Why not just make them ordinary subs, then? Ordinary subs can't be called
by method invocation, and we want to call these routines that way. Furthermore,
if your base class does define an ordinary method named BUILD
or DESTROY, it can serve as the default BUILD
or DESTROY for all derived classes that don't declare their
own submethods. (All public methods are virtual in Perl, but some are
more virtual than others.)

You might be saying to yourself, "Wait, private methods aren't virtual.
Why not just use a private method for this?" It's true that private
methods aren't virtual, because they aren't in fact methods at all.
They're just ordinary subroutines in disguise. They have nothing to
do with inheritance. By contrast, submethods are all about presenting
a unified inherited interface with the option of either inheriting
or not inheriting the implementation of that interface, at
the discretion of the class doing the implementing.

So the bottom line is that submethods allow you to override an inherited
implementation for the current class without overriding the default
implementation for other classes. But in any case, it's still using
a public interface, called as an ordinary method call, from anywhere
in your program that has an object of your type.

Or a class of your type. The default new constructor is
an ordinary class method in class Object, so it's inherited
by all classes that don't define their own new. But when
you write your own new, you need to decide whether your
constructor should be inherited or not. If so, that's good, and you
should declare it as a method. But if not, you should declare it as
a submethod so that derived classes don't try to use it erroneously
instead of the default Object.new().

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

In Perl 6, "attributes" are what we call the instance variables of an
object. (We used that word to mean something else in Perl 5--we're now
calling those things "traits" or "properties".)

As with classes and methods, attribute declarations are apparently declarative.
Underneath they actually call a method in the metaclass to install the
new definition. The Perl 6 implementation of attributes is not based
on a hash, but on something more like a symbol table. Attributes are
stored in an opaque datatype rather like a struct in C, or an array
in Perl 5--but you don't know that. The datatype is opaque in the sense
that you shouldn't care how it's laid out in memory (unless you have
to interface with an outside data structure--like a C struct). Do not
confuse opacity with encapsulation. Encapsulation only hides the object's
implementation from the outside world. But the object's structure is
opaque even to the class that defines it.

One of the large benefits of this is that you can actually take a C or
C++ data structure and wrap accessor methods around it without having
to copy anything into a different data structure. This should speed
up things like XML parsing.

In order to provide this opaque abstraction layer, attributes are not
declared as a part of any other data structure. Instead, they are modeled
on real variables, whose storage details are implicitly delegated to
the scope in which they are declared. So attributes are declared as
if they were normal variables, but with a strange scope and lifetime
that is neither my nor our. (That scope is,
of course, the current object, and the variable lives as long as the
object lasts.) The class will implicitly store those attributes in a
location distinct from any other class's attributes of the same name,
including any base or derived class. To declare an attribute variable,
declare it within the class definition as you would a my
variable, but use the has declarator instead of my:

class Dog is Mammal {
has $.tail;
has @.legs;
...
}

The has declarator was chosen to remind people that attributes
are in a "HASA" relationship to the object rather than an "ISA" relationship.

The other difference from normal variables is that attributes have a
secondary sigil that indicates that they are associated with methods.
When you declare an attribute like $.tail, you're also
implicitly declaring an accessor method of the same name, only without
the $ on the front. The dot is there to remind you that
it's also a method call.

As with other declarations, you may add various traits to an attribute:

has $.dogtag is rw;

If you want all your attributes to default to "rw", you
can put the attribute on the class itself:

class Coordinates is rw {
has int $.x;
has int $.y;
has int $.z;
}

Essentially, it's now a C-style struct, without having to introduce an
ugly word like "struct" into the language. Take that, C++. :-)

You can also assign to a declaration:

has $.master = "TheDamian";

Well, actually, this looks like an assignment, but it isn't. The effect
of this is to establish a default; it is not executed at runtime. (Or
more precisely, it runs when the class closure is executed by the metaclass,
so it gets evaluated only once and the value is stored for later use
by real instances. More below.)

The attribute behaves just like an ordinary variable within the class's
instance methods. You can read and write the attributes just like ordinary
variables. (It is, however, illegal to refer to an instance attribute
variable (that is, a "has" variable) from within a class
method. Class methods may only access class attributes, not instance
attributes. See below.)

Bare attributes are automatically hidden from the outside world because
their sigiled names cannot be seen outside the class's package. This
is how Perl 6 enforces encapsulation. Outside the class the only
way to talk about an attribute is through accessor methods. Since public
methods are always virtual in Perl, this makes attribute access virtual
outside the class. Always. (Unless you give the optimizer enough hints
to optimize the class to "final". More on that later.)

In other words, only the class itself is allowed to know whether this
attribute is, in fact, implemented by this class. The class may also
choose to ignore that fact, and call the abstract interface, that is,
the accessor method, in which case it might actually end up calling
some derived class's overriding method, which might in turn call back
to this class's accessor as a super method. (So in general, an accessor
method should always refer to its actual variable name rather than the
accessor method name to avoid infinite recursion.)

You may write your own accessor methods around the bare attributes, but
if you don't, Perl will generate them for you based on the declaration
of the attribute variable. The traits of the generated method correspond
directly to the traits on the variable.

By default, a generated accessor is read-only (because by default any
method is read-only). If you mark an attribute with the trait "is
rw" though, the corresponding generated accessor will also be
marked "is rw", meaning that it can be used as an lvalue.

In any event, even without "is rw" the attribute variable
is always writable within the class itself (unless you apply the trait
is constant to it).

As with private classes and methods, attributes are declared private
using a colon on the front of their names. As with any private method,
a private accessor is completely ignored outside its class (or, by extension,
the classes trusted by this class).

To carry the separate namespace idea through, we incorporate the colon
as the secondary sigil in declarations of private attributes:

has $:x;

Then we can get rid of the verbose is private altogether.
Well, it's still there as a trait, but the colon implies it, and is
required anyway.) And we basically force people to document the private/public
distinction every place they reference $:x instead of $.x,
or $obj.:meth instead of $obj.meth.

We've seen secondary sigils before in earlier Apocalypses. In each case
they're associated with a bizarre usage of some sort. So far we have:

As a form of the dreaded "Hungarian notation", secondary sigils are not
introduced lightly. We define secondary sigils only where we deem instant
recognizability to be crucial for readability. Just as you should never
have to look at a variable and guess whether it's a true global, you
should never have to look at a method and guess which variables are
attributes and which ones are variables you just happen to be in the
lexical scope of. Or which attributes are public and which are private.
In Perl 6 it's always obvious--at the cost of a secondary sigil.

We do hereby solemnly swear to never, never, ever add tertiary sigils.
You have been warned.

You can set default values on attributes by pseudo-assignment to the
attribute declaration:

has Answer $.ans = 42;

These default values are associated as "build" traits of
the attribute declaration object. When the BUILD submethod
is initializing a new object, these prototype values are used for uninitialized
attributes. The expression on the right is evaluated immediately at
the point of declaration, but you can defer evaluation by passing a
closure, which will automatically be evaluated at the actual initialization
time. (Therefore, to initialize to a closure value, you have to put
a closure in a closure.)

Here's the difference between those three approaches. Suppose you say:

When the object is eventually constructed, $.ans1 will be
initialized to 0, while $.ans2 will be initialized
to 42. (That's because the closure binds $defaultanswer
to the current variable, which still presumably has the value 42 by
the time the BUILD routine initializes the new object,
even though the lexical variable "$defaultanswer" has supposedly
gone out of scope by the time the object is being constructed. That's
just how closures work.)

And $.ans3 will be initialized not to 42, but to a closure
that, if you ever call it, will also return 42. So since the accessor
$obj.ans3() returns that closure, $obj.ans3().()
will return 42.

The default value is actually stored under the "build" trait,
so this:

has $.x = calc($y);

is equivalent to this:

has $.x is build( calc($y) );

and this:

has $.x = { calc($y) };

is equivalent to either of these:

has $.x is build( { calc($y) } );
has $.x will build { calc($y) };

As with all closure-valued container traits, the container being declared
(the $.x variable in this case) is passed as the topic
to the closure (in addition to being the target that will be initialized
with the result of the closure, because that's what build
does). In addition to the magical topic, these build traits are also
magically passed the same named arguments that are passed to the BUILD
routine. So you could say

has $.x = { calc($^y) };

to do a calculation based on the :y(582) parameter originally
passed to the constructor. Or rather, that will be passed to the constructor
someday when the object is eventually constructed. Remember we're really
still at class construction time here.

As with other initializers, you can be more specific about the time at
which the default value is constructed, as long as that time is earlier
than class construction time:

In general, class attributes are just package or lexical variables. If
you define a package variable with a dot or colon, it autogenerates
an accessor for you just as it does for an ordinary attribute:

The implicit invocant of these implicit accessors has a "squinting" type--it
can either be the class or an object of the class. (Declare your own
accessors if you have a philosophical reason for forcing the type one
way or the other.)

The disadvantage of using "our" above is that both
of these are accessible from outside the class via their package name
(though the private one is Officially Ignored, and cannot be named simply
by saying %MyClass:::cache because that syntax is specifically
disallowed).

then the same pair of accessors are generated, but the variables themselves
are visible only within the class block. If you reopen the class in
another block, you can only see the accessors, not the bare variables.
This is probably a feature.

Generally speaking, though, unless you want to provide public accessors
for your class attributes, it's best to just declare them as ordinary
variables (either my or state variables) to
prevent confusion with instance attributes. It's a good policy not to
declare any public accessors until you know you need them. They are,
after all, part of your contract with the outside world, and the outside
world has a way of holding you to your contracts.

The basic idea here is to remove the drudgery of creating objects. In
addition we want object creation and cleanup to work right by default.
In Perl 5 it's possible to make recursive construction and destruction
work, but it's not the default, and it's not easy.

Perl 5 also confused the notions of constructor and initializer. A constructor
should create a new object once, then call all the appropriate initializers
in the inheritance tree without recreating the object. The initializer
for a base class should be called before the initializer for any class
derived from it.

The initializer for a class is always named BUILD. It's
in uppercase because it's usually called automatically for you at construction
time.

As with Perl 5, a constructor is only named "new" by convention,
and you can write a constructor with any name you like. However, in
Perl 6, if you do not supply a "new" method, a generic
one will be provided (by inheritance from Object, as it
happens).

The arguments for the default constructor are always named arguments,
hence the *%_ declaration to collect all those pairs and
pass them on to bless.

You'll note also that bless is no longer a subroutine but
a method call, so it's now impossible to omit the class specification.
This makes it easier to inherit constructors. You can still bless any
reference you could bless in Perl 5, but where you previously used a
function to do that:

# Perl 5 code...
return bless( {attr => "hi"}, $class );

in Perl 6 you use a method call:

# Perl 6 code...
return $class.bless( {attr => "hi"} );

However, if what you pass as the first argument isn't a reference, bless
is going to construct an opaque object and initialize it. In a sense,
bless is the only real constructor in Perl 6. It first
makes sure the data structure is created. If you don't supply a reference
to bless, it calls CREATE to create the object. Then it
calls BUILDALL to call all the initializers.

The signature of bless is something like:

method bless ($class: $candidate, *%_)

The 0 candidate indicates the built-in opaque type. If you're
really strange in the head, you can think of the "0" as
standing for "0paque". Or it's the "zero" object, about
which we know zip. Whatever tilts your windmill...

In any event, strings are reserved for other object layouts. We could
conceivably have things like:

return $class.bless("Cstruct", *%_);

So as it happens, 0 is short for the layout "P6opaque".

Any additional arguments to .bless are automatically passed
on to CREATE and BUILDALL. But note that these
must be named arguments. It could be argued that the only
real purpose for writing a .new constructor in Perl 6 is
to translate different positional argument signatures into a unified
set of named arguments. Any other initialization common to all constructors
should be done within BUILD.

Oh, the invocant of .bless is either a class or an object
of the class, but if you use an object of the class, the contents of
that object are not automatically used to prototype the new
object. If you wish to do that, you have to do it explicitly by copying
the attributes:

$obj.bless(0, *%$obj)

(That is just a specific application of the general principle that if
you treat any object like a hash, it will behave like one, to the extent
that it can. That is, %$obj turns the attributes into key/value
pairs, and passes those as arguments to initialize the new object. Note
that %$obj includes the private attributes when used inside
the class, but not outside.)

Just because .bless allows an object to be used for a class
doesn't mean your new constructor has to do the same. Some
folks have philosophical issues with mixing up classes and objects,
and it's fine to disallow that on the constructor level. In fact, you'll
note that the default .new above requires a Class
as its invocant. Unless you override it, it doesn't allow an object
for the constructor invocant. Go thou and don't likewise.

Note the order of the two hash arguments to bless. This
gives the supplied attribute values precedence over the copied attribute
values, so that you can change some of the attributes en passant,
if you like. That's because we're passing the two flattened hashes as
arguments to .bless and Perl 6's named argument binding
mechanism always picks the first argument that matches, not
the last. This is opposite of what happens when you use the Perl 5 idiom:

CREATE is called when you don't want to use an existing
data structure as the candidate for your object. In general you won't
define CREATE because the default CREATE does
all the heavy magic to bring an opaque object into existence. But if
you don't want an opaque object, and you don't care to write all your
constructors to create the data structure before calling .bless,
you can define your own CREATE submethod, and it will override
the standard one for all constructors in the class.

After the data structure is created, it must be populated by each of
the participating classes (and roles) in the proper order. The BUILDALL
method is called upon to do this. The default BUILDALL
is usually correct, so you don't generally have to override it. In essence,
it delegates the initialization of parent classes to the BUILDALL
of the parent classes, and then it calls BUILD on the current
class. In this way the pieces of the object are assembled in the correct
order, from least derived to most derived.

For each class BUILDALL calls on, if the arguments contain
a pair whose key is that class name, it passes the value of the pair
as its argument to that class's BUILDALL. Otherwise it
passes the entire list. (There's not much ambiguity there--most classes
and roles will start with upper case, while most attribute names start
with lower case.)

That occurs so frequently that there's a shorthand available in the signature
declaration. You can put the attributes (distinguished by those secondary
sigils, you'll recall) right into the signature. The following means
essentially the same thing, without repeating the names:

submethod BUILD (+$.tail, +@:legs, *%extraargs) {...}

It's actually unnecessary to declare the *%extraargs parameter.
If you leave it out, it will default to *%_ (but only on
methods and submethods--see the section on Interface Consistency later).

You may use this special syntax only for instance attributes, not class
attributes. Class attributes should generally not be reinitialized every
time you make a new object, after all.

If you do not declare a BUILD routine, a default routine
will be supplied that initializes any attributes whose names correspond
to the keys of the argument pairs passed to it, and leaves the other
attributes to default to whatever the class supplied as the default,
or undef otherwise.

In any event, the assignment of default attribute values happens automatically.
For any attribute that is not otherwise initialized, the attribute declaration's
"build" property is evaluated and the resulting value copied
in to the newly created attribute slot. This happens logically at the
end of the BUILD block, so we avoid running initialization
closures unnecessarily. This implicit initialization is based not on
whether the attribute is undefined, but on whether it was initialized
earlier in BUILD. (Otherwise we could never explicitly
create an attribute with an undefined value.)

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

you have to repeat the type. That's not a big deal for a small typename,
but sometime typenames are a lot longer. Plus you'd like to get rid
of the redundancy, just because it's, like, redundant. So there's a
variant on the dot operator that looks a lot like a dot assignment operator:

my Dog $spot .= new(...)

It doesn't really quite fit the assignment operator rule though. If it
did, it'd have to mean:

my Dog $spot = $spot.new(...)

which doesn't quite work, because $spot is undefined. What
probably happens is that the my cheats and puts a version
of undef in there that knows it should dispatch to the
Dog class if you call .self:new() on it. Anyway,
we'll make it work one way or another, so that it becomes the equivalent
of:

my Dog $spot = Dog.new(...)

The alternative is to go the C++ route and make new a reserved
word. We're just not going to do that.

Note that an attribute declaration of the form

has Tail $wagger .= new(...)

might not do what you want done when you want it done, if what you want
done is to create a new Dog object each time an object
is built. For that you'd have to say:

has Tail $wagger = { .new(...) }

or equivalently,

has Tail $wagger will build { .new(...) }

But leaving aside such timing issues, you should generally think of the
.= operator more as a variant on . than a
variant on +=. It can, for instance, turn any non-mutating
method call into a mutating method:

@array.=sort; # sort @array in place
.=lc; # lowercase $_ in place

This presumes, of course, that the method's invocant and return value
are of compatible types. Some classes will wish to define special in-place
mutators. The syntax for that is:

method self:sort (Array @a is rw) {...}

It is illegal to use return from such a routine, since the
invocant is automatically returned. If you do not declare the invocant,
the default invocant is automatically considered "rw".
If you do not supply a mutating version, one is autogenerated for you
based on the corresponding copy operator.

The DESTROYALL routine is the counterpart to the BUILDALL
routine. Similarly, the default definition is normally sufficient for
the needs of most classes. DESTROYALL first calls DESTROY
on the current class, and then delegates to the DESTROYALL
of any parent classes. In this way the pieces of the object are disassembled
in the correct order, from most derived to least derived.

As with Perl 5, all the memory deallocation is done for you, so you really
only need to define DESTROY if you have to release external
resources such as files.

Since DESTROY is the opposite of BUILD, if
any attribute declaration has a "destroy" property, that
property (presumably a closure) is evaluated before the main block of
DESTROY. This happens even if you don't declare a DESTROY.

(The "build" and "destroy" traits are the only
way for roles to let their preferences be made known at BUILD
and DESTROY time. It follows that any role that does not
define an attribute cannot participate in building and destroying except
by defining a method that BUILD or DESTROY
might call. In other words, stateless roles aren't allowed to muck around
with the object's state. This is construed as a feature.)

Single dispatch looks up which method to run solely on the basis of the
type of the first argument, the invocant. A single-dispatch call distinguishes
the invocant syntactically (unlike a multiple-dispatch call, which looks
like a subroutine call, or even an operator.)

Basically, anything can be an invocant as long as it fills the Dispatch
role, which provides a .dispatcher method. This includes
ordinary objects, class objects, and (in some cases) even varieties
of undef that happen to know what class of thing they aren't
(yet).

Simple single dispatch is specified with the dot operator, or its indirect
object equivalent:

There are variants on the dot form indicated by the character after the
dot. (None of these variants allows indirect object syntax.) The private
dispatcher only ever dispatches to the current class or its proxies,
so it's really more like a subroutine call in disguise:

It is an error to use .: unless there is a correspondingly
named "colon" method in the appropriate class, just as it is an error
to use . when no method can be found of that name. Unlike
the .: operator, which can have only one candidate method,
the . operator potentially generates a list of candidates,
and allows methods in that candidate list to defer to subsequent methods
in other classes until a candidate has been found that is willing to
handle the dispatch.

In addition to the .: and .= operators, there
are three other dot variants that can be used if it's not known how
many methods are willing to handle the dispatch:

$object.?meth(@args) # calls method if there is one
.?meth(@args) # unary form
$object.*meth(@args) # calls all methods (0 or more)
.*meth(@args) # unary form
$object.+meth(@args) # calls all methods (1 or more)
.+meth(@args) # unary form

The .* and .+ versions are generally only useful
for calling submethods, or methods that are otherwise expected to work
like submethods. They return a list of all the successful return values.
The .? operator either returns the one successful result,
or undef if no appropriate method is found. Like the corresponding regex
modifiers, ? means "0 or 1", while * means
"0 or more", and + means "1 or more". Ordinary .
means "exactly one". Here are some sample implementations, though of
course these are probably implemented in C for maximum efficiency:

This one you can quit early by saying "last METHOD". Notice
that both of these dispatchers cheat by calling a method as if it were
a sub. You may only do that by taking a reference to the method, and
calling it as a subroutine, passing the object as the first argument.
This is the only way to call a virtual method non-virtually in Perl.
If you try to call a method directly as a subroutine, Perl will ignore
the method, look for a subroutine of that name elsewhere, probably not
find it, and complain bitterly. (Or find the wrong subroutine, and execute
it, after which you will complain bitterly.)

We snuck in an example the new gather/take
construct. It is still somewhat conjectural.

Perl 5 supplies a pseudoclass, SUPER::, that redirects dispatch
to a parent class's method. That's often the wrong thing to do, though,
in part because under MI you may have more than one parent class, and
also because you might have sibling classes that also need to have the
given method triggered. Even if SUPER is smart enough to
visit multiple parent classes, and even if all your classes cooperate
and call SUPER at the right time, the depth first order
of visitation might be the wrong order, especially under diamond inheritance.
Still, if you know that your parent classes use SUPER,
or you're calling into a language with SUPER semantics
(such as Perl 5) then you should probably use SUPER semantics
too, or you'll end up calling your parent's parents in duplicate. However,
since use of SUPER is slightly discouraged, we Huffman
code it a bit longer in Perl 6. Remember the *%opt parameters
to the dispatchers above? That comes in as a parameterized pseudoclass
called WALK.

$obj.*WALK[:super]::method(@args)

That limits the call to only those immediate super classes that define
the method. Note the star in the example. If you really want the Perl
5 semantics, leave the star out, and you'll only get the first existing
parent method of that name. (Why you'd want that is beyond me.)

Actually, we'll probably still allow SUPER:: as a shorthand
for WALK[:super]::, since people will just hack it in anyway
if we don't provide it...

If you think about it, every ordinary dispatch has an implicit WALK
modifier on the front that just happens to default to WALK[:canonical].
That is, the dispatcher looks for methods in the canonical order. But
you could say WALK[:depth] to get Perl 5's order, or you
could say WALK[:descendant] to get an order approximating
the order of construction, or WALK[:ascendant] to get an
order approximating the order of destruction. You could say WALK[:omit(SomeClass)]
to call all classes not equivalent to or derived from SomeClass.
For instance, to call all super classes, and not just your immediate
parents, you could say WALK[:omit(::_)] to skip the current
lexical class or anything derived from it.

But again, that's not usually the right thing to do. If your base classes
are all willing to cooperate, it's much better to simply call

$obj.method(@args)

and then let each of the implementations of the method defer to the next
one when they're done with their part of it. If any method says "next
METHOD", it automatically iterates the loop of the dispatcher
and finds the next method to dispatch to, even if that method comes
from a sibling class rather than a parent class. The next method is
called with the same arguments as originally supplied.

That presupposes that the entire set of methods knows to call "next"
appropriately. This is not always the case. In fact, if they don't all
call next, it's likely that none of them does. And maybe just knowing
whether or not they do is considered a violation of encapsulation. In
any case, if you still want to call all the methods without their active
cooperation, then use the star form:

$obj.*method(@args)

Then the various methods don't have to do anything to call the next method--it
happens automatically by default. In this case a method has to do something
special if it wants to stop the dispatch. Naturally, that something
is to call "last METHOD", which terminates the dispatch
loop early.

Now, sometimes you want to call the next method, but you want to change
the arguments so that the next method doesn't get the original argument
list. This is done with deep magic. If you use the call
keyword in an ordinary (nonwrapper) method, it steals the rest of the
dispatch list from the outer loop and redispatches to the next method
with the new arguments:

@retvals = call(@newargs)
return @retvals;

And unlike with "next METHOD", control returns to this method
following the call. It returns the results of the subsequent method
calls, which you should return so that your outer dispatcher can add
them to the return values it already gathered.

Note that "next METHOD" and "last METHOD" can
typically be spelt "next" and "last" unless
they are in an inner loop.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

By default the various dot operators call a method on a single object,
even if it ends up calling multiple methods for that object. Since a
method call is essentially a unary postfix operator, however, you can
use it as a hyper operator on a list of objects:

@object».meth(@args) # Call one for each or fail
@object».?meth(@args) # Call one for each if available
@object».*meth(@args) # Call all available for each
@object».+meth(@args) # Call one or more for each

Note that with the last two, if a method uses "last METHOD",
it doesn't bomb out of the "hyper" loop, but just goes on to the next
entry. One can always bomb out of the hyperloop with a real exception,
of course. And maybe with "last HYPER", depending on how
hyper's implicit iteration is implemented.

If you want to use an array for serial rather than parallel method calling,
see Delegation, which lets you set up cascading handlers.

The WALKCLASS and WALKMETH routines used in
the sample dispatch code need to cache their results so that every dispatch
doesn't have to traverse the inheritance tree again, but just consult
the preconstructed list in order. However, if there are changes to any
of the classes involved, then someone needs to call the appropriate
cache clear method to make sure that the inheritance is recalculated.

Note that :method(Str) selects classes that merely have
methods declared, not necessarily defined. A declaration without a definition
probably implies that they intend to autoload a definition, so we should
call the stub anyway. In fact, Perl 6 differentiates an AUTOMETHDEF
from AUTOLOAD. AUTOLOAD works as it does in
Perl 5. AUTOMETHDEF is never called unless there is already
a declaration of the stub (or equivalently, AUTOMETH faked
a stub.)

It would be possible to just define everything in terms of WALKCLASS,
but that would imply looking up each method name twice, once inside
WALKCLASS to see if the method exists in the current class,
and once again outside in order to call it. Even if WALKCLASS
caches the cache list, it wouldn't cache the derived method list, so
it's better to have a separate cache for that, controlled by WALKMETH,
since that's the common case and has to be fast.

(Again, this is all abstract, and is probably implemented in gloriously
grungy C code. Nevertheless, you can probably call WALKCLASS
and WALKMETH yourself if you feel like writing your own
dispatcher.)

Multiple dispatch is based on the notion that methods often mediate the
relationships of multiple objects of diverse types, and therefore the
first object in the argument list should not be privileged over other
objects in the argument list when it comes to selecting which method
to run. In this view, methods aren't subservient to a particular class,
but are independent agents. A set of independent-minded, identically
named methods use the class hierarchy to do pattern matching on the
argument list and decide among themselves which method can best handle
the given set of arguments.

The Perl approach is, of course, that sometimes you want to distinguish
the first invocant, and sometimes you don't. The interaction of these
two approaches gets, um, interesting. But the basic notion is to let
the caller specify which approach is expected, and then, where it makes
sense, fall back on the other approach when the first one fails. Underlying
all this is the Principle of Least Surprise. Do not confuse this with
the Principle of Zero Surprise, which usually means you've just swept
the real surprises under some else's carpet. (There's a certain amount
of surprise you can't go below--the Heisenberg Uncertainty Principle
applies to software too.)

With traditional multimethods, all methods live in the same global namespace.
Perl 6 takes a different approach--we still keep all the traditional
Perl namespaces (lexical, package, global) and we still search for names
the same way (outward through the lexical scopes, then the current package,
then the global * namespace; or upward in the class hierarchy).
Then we simply claim that, under multiple dispatch, the "long name"
of any multi routine includes its signature, and that visibility is
based on the long name. So an inner or derived multi only hides an outer
or base multi of the same name and the same signature. (Routines
not declared "multi" still hide everything in the traditional
fashion.)

To put it another way, the multiple dispatch always works when both the
caller and the callee agree that that's how it should work. (And in
some cases it also works when it ought to work, even if they don't agree--sort
of a "common law" multimethod, as it were...)

A callee agrees to the multiple dispatch "contract" by including the
word "multi" in the declaration of the routine in question.
It essentially says, "Ordinarily this would be a unique name, but it's
okay to have duplicates of this name (the short name) that are differentiated
by signatures (the long name)."

Looking at it from the other end, leaving the "multi" out
says "I am a perfect match for any signature--don't bother looking any
further outward or upward." In other words, the standard non-multi semantics.

You may not declare a multi in the same scope as a non-multi. However,
as long as they are in different scopes, you can have a single non-multi
inside a set of multis, or a set of multis inside a single non-multi.
You can even have a set of multis inside a non-multi inside a set of
multis. Indeed, this is how you hide all the outer multis so that only
the inner multi's long names are considered. (And if no long name matches,
you get the intermediate non-multi as a kind of backstop.) The same
policy applies to both nested lexical scopes and derived subclasses.

Actually, up till now we've been oversimplifying the concept of "long
name" slightly. The long name includes only that part of the signature
up to the first colon. If there is no colon, then the entire signature
is part of the long name. (You can have more colons, in which case the
additional arguments function as tie breakers if the original set of
long names is insufficient to prevent a tie.)

If you want a multi that is visible in all namespaces (that don't hide
the long name), then declare the name in the global name space, indicated
in Perl 6 with a *. Most of the so-called "built-ins" are
declared this way:

If you declare a method with multi, then that method hides
any base class method with the same long name. It does not hide methods
with the same short name but a different signature when called as a
multimethod. (It does hide methods when called under single dispatch,
in which case the first invocant is treated as the only invocant
regardless of where you put the colon. Just because a method is declared
with multi doesn't make it invisible to single dispatch.)

Unlike a regular method declaration, there is no implied invocant in
the syntax of a multi method. A method declared as multi must
declare all its invocants so that there's no ambiguity as to the meaning
of the first colon. With a multi method, it always means the end of
the long name. (With a non-multi, it always means that the optional
invocant declaration is present.)

Submethods may be declared with multi, in which case visibility
works the same as for ordinary methods. However, a submethod has the
additional constraint that the first invocant must be an exact class
match. Which effectively means that a submethod is first single dispatched
to the class, and then the appropriate submethod within that class is
selected, ignoring any other class's submethods of the same name.

It is not likely that Perl 6.0.0 will support multiple dispatch on named
arguments, but only on positional arguments. Since all the extra arguments
to a BUILD routine come in as named arguments, you probably
can't usefully multi a BUILD (yet). However, we should
not do anything that precludes multiple BUILD submethods
in the future. Which means we should probably enforce the presence of
a colon before the first named argument declaration in any multi signature,
so that the semantics don't suddenly change if and when we start supporting
multiple dispatch that includes named arguments as part of the long
name.

As we mentioned, multiple dispatch is enabled by agreement of both caller
and callee. From the caller's point of view, you invoke multiple dispatch
simply by calling with subroutine call syntax instead of method call
syntax. It's then up to the dispatcher to figure out which of the arguments
are invocants and which ones are just options. (In the case where the
innermost visible subroutine is declared non-multi, this degenerates
to the Perl 5 semantics of subroutine calls.) This approach lets you
refactor a simple subroutine into a more nuanced set of subroutines
without changing how the subroutines are called at all. That makes this
sort of refactoring drop-dead simple. (Or at least as simple as refactoring
ever gets...)

It's a little harder to refactor between single dispatch and multiple
dispatch, but a good argument could be made that it should
be harder to do that, because you're going to have to think through
a lot more things in that case anyway.

Anyway, here's the basic relationship between single dispatch and multiple
dispatch. Single dispatch is more familiar, so we'll discuss multiple
dispatch first.

Whenever you make a call using subroutine call syntax, it's a candidate
for multiple dispatch. A search is made for an appropriate subroutine
declaration. As in Perl 5, this search goes outward through the lexical
scopes, then through the current package and on to the global namespace
(represented in Perl 6 with an initial * for the "wildcard" package
name). If the name found is not a multi, then it's a good old-fashioned
sub call, and no multiple dispatch is done. End of story.

However, if the first declaration we come to is a multi, then lots of
interesting stuff happens. (Fortunately for our performance, most of
this interesting stuff can happen at compile time, or upon first use.)
The basic idea is that we will collect a complete list of candidates
before we decide which one to call.

So the search continues outward, collecting all sub declarations with
the same short name but different long names. (We can ignore outer declarations
that are hidden by an inner declaration with the same long name.) If
we run into a scope with a non-multi declaration, then we're done generating
our candidate list, and we can skip the next paragraph.

After going all the way out to the global scope, we then examine the
type of the first argument as if we were about to do single dispatch
on it. We then visit any classes that would have been single dispatched,
in most-derived to least-derived order, and for each of those classes
we add into our candidate list any methods declared multi, plus all
the single invocant methods, whether or not they were declared multi!
In other words, we just add in all the methods declared in the class
as a subset of the candidates. (There are reasons for this that we'll
discuss below.) Anyway, just as with nested lexical scopes, if two methods
have the same long name, the more derived one hides the less derived
one. And if there's a class in which the method of the same short name
is not declared multi, it serves as a "stopper", just as a non-multi
sub does in a lexical scope. (Though that "stopper" method can of course
redispatch further up the inheritance tree, just as a "stopper" lexical
sub can always call further outward if it wants to.)

Now we have our list of candidates, which may or may not include every
sub and method with the same short name, depending on whether we hit
a "stopper". Anyway, once we know the candidate list, it is sorted into
order of distance from the actual argument types. Any exact match on
a parameter type is distance 0. Any miss by a single level of derivation
counts as a distance of 1. Any violation of a hard constraint (such
as having too many arguments for the number of parameters, or violating
a subtype check on a type that does constraint checking, or missing
the exact type on a submethod) is effectively an infinite distance,
and disqualifies the candidate completely.

Once we have our list of candidates sorted, we simply call the first
one on the list, unless there's more than one "first one" on the list,
in which case we look to see if one of them is declared to be the default.
If so, we call it. If not, we die.

So if there's a tie, the default routine is in charge of subsequent behavior:

If you use the dot notation, you are explicitly calling single dispatch.
By default, if single dispatch doesn't find a suitable method, it does
a "failsoft" to multiple dispatch, pretending that you called a subroutine
with the invocant passed as the first argument. (Multiple dispatch doesn't
need to failsoft to single dispatch since all single dispatch methods
are included as a subset of the multiple dispatch candidates anyway.)

This failsoft behavior can be modified by lexically scoped pragma. If
you say

use dispatch :failhard

then single dispatch will be totally unforgiving as it is in Perl 5.
Or you can tell single dispatch to go away:

use dispatch :multi

in which case all your dot notation is treated as a sub call. That is,
any

$obj.method(1,2,3)

in the lexical scope acts like you'd said:

method($obj,1,2,3)

If single dispatch locates a class that defines the method, but the method
in question turns out to be a set of one or more multi methods, then,
the single dispatch fails immediately and a multiple dispatch is done,
with the additional constraint that only multis within that class are
considered. (If you wanted the first argument to do loose matching as
well, you should have called it as a multimethod in the first place.)

If you use indirect object syntax with an explicit colon, it is exactly
equivalent to dot notation in its semantics.

However, one-argument subs are inherently ambiguous, because Perl 6 does
not require the colon on indirect objects without arguments. That is,
if you say:

print $fh

it's not clear whether you mean

$fh.print

or

print($fh)

As it happens, we've defined the semantics so that it doesn't matter.
Since all single invocant methods are included automatically in multimethod
dispatch, and since multiple dispatch degenerates to single dispatch
when there's only one invocant, it doesn't matter which way your write
it. The effect is the same either way. (Unless you've defined your own
non-multi print routine in a surrounding lexical scope. But then, if
you've done that, you probably did it on purpose precisely
because you wanted to disable the default dispatch semantics.)

Within the context of a multimethod dispatch, "next METHOD"
means to try the next best match, if unambiguous, or else the marked
default method. From within the default method it means just pick the
next in the list even if it's ambiguous. The dispatch list is actually
kept in @CALLER::methods, which is a list of pairs, the
key of each indicating the "distance" rating, and the value of each
containing a reference to the method to call (as a sub ref).

If you want to directly access the attributes of a class, your multi
must be declared within the scope of that class. Attributes are never
directly visible outside a class. This makes it difficult to write an
efficient multimethod that knows about the internals of two different
classes. However, it's possible for private accessors to be visible
outside your class under one condition. If your class declares that
another class is trusted, that other class can see the private accessors
of your class. If the other class declares that you are trusted, then
you can see its private accessor methods. The trust relationship is
not necessarily symmetrical. This lets you have an architecture where
classes by and large don't trust each other, but they all trust a single
well-guarded "multi-plexor" class that keeps everyone else
in line.

The syntax for trusting another class is simply:

class MyClass {
trusts Yourclass;
...
}

It's not clear whether roles should be allowed to grant trust. In the
absence of evidence to the contrary, I'm inclined to say not. We can
always relax that later if, after many large, longitudinal, double-blind
studies, it turns out to be both safe and effective.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

In Perl 5 overloading was this big special deal that had to have special
hooks inserted all over the C code to catch various operations on overloaded
types and do something special with them. In Perl 6, that just all falls
out naturally from multiple dispatch. The only other part of the trick
is to consider operators to be function calls in disguise. So in Perl
6 the real name of an operator is composed of a grammatical context
identifier, a colon, and then the name of the operator as you usually
see it. The common context identifiers are "prefix", "infix", "postfix",
"circumfix", and "term", but there are others.

Perl 5 had special key names representing stringification and numification.
In Perl 6 these naturally fall out if you define:

method prefix:+ () {...} # what we do in numeric context
method prefix:~ () {...} # what we do in string context

Likewise you can define what to return in boolean context:

method prefix:? () {...} # what we do in boolean context

Integer context is, of course, just an ordinary method:

method int () {...} # what we do in integer context

These can be defined as normal methods since single-invocant multi subs
degenerate to standard methods anyway. C++ programmers will tend to
feel comfy defining these as methods. But others may prefer to declare
them as multi subs for consistency with binary operators. In which case
they'd look more like this:

multi sub *prefix:+ (Us $us) {...} # what we do in numeric context
multi sub *prefix:~ (Us $us) {...} # what we do in string context
multi sub *prefix:? (Us $us) {...} # what we do in string context
multi sub *prefix:int (Us $us) {...} # what we do in integer context

The "is commutative" trait installs an additional autogenerated
sub with the invocant arguments reversed, but with the same semantics
otherwise. So the declaration above effectively autogenerates this:

multi sub infix:+ (Them $them, Us $us) {...}

Of course, there's no need for that if the two arguments have the same
type. And there might not actually be an autogenerated other subroutine
in any case, if the implementation can be smart enough to simply swap
the two arguments when it needs to. However it gets implemented, note
that there's no need for Perl 5's "reversed arguments flag" kludge,
since we reverse the parameter name bindings along with the types. Perl
5 couldn't do that because it had no control of the signature from the
compiler's point of view.

See Apocalypse 6 for much more on the definition of user-defined operators,
their precedence, and their associativity. Some of it might even still
be accurate.

Objects have many kinds of relationships with other objects. One of the
pitfalls of the early OO movement was to encourage people to model many
relationships with inheritance that weren't really "isa" relationships.
Various languages have sought to redress this deficiency in various
ways, with varying degrees of success. With Perl 6 we'd like to back
off a step and allow the user to define abstract relationships between
classes without committing to a particular implementation.

More specifically, we buy the argument of the Traits
paper that classes should not be used both to manage objects and
to manage code reuse. It needs to be possible to separate those concerns.
Since a lot of the code that people want to reuse is that which manages
non-isa object relationships, that's what we should abstract out from
classes.

That abstraction we are calling a role. Roles can encompass both interface
and implementation of object relationships. A role without implementation
degenerates to an interface. A role without interface degenerates to
privately instantiated generics. But the typical role will provide both
interface and at least a default implementation.

Unlike the Traits paper, we will allow state as part of our implementation.
This is necessary if we are to abstract out the delegation decision.
We feel that the decision to delegate rather than compose a sub-object
is a matter of implementation, and therefore that decision should be
encapsulated (or at least be allowed to be encapsulated) in a role.
This allows you to refactor a problem by redefining one or more roles
without having to doctor all the classes that make use of those roles.
This is a great way to turn your huge, glorious "god object" into a
cooperating set of objects that know how to delegate to each other.

As in the Traits paper, roles are composed at class construction time,
and the class composer does some work to make sure the composed class
is not unintentionally ambiguous. If two methods of the same name are
composed into the same class, the ambiguity will be caught. The author
of the class has various remedies for dealing with this situation, which
we'll go into below.

From the standpoint of the typical user, a role just looks like a "smart"
include of a "partial class". They're smart in that roles have to be
well behaved in certain respects, but most of the time the naive user
can ignore the power of the abstraction.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

A role may not inherit from a class. It may be composed of other roles,
however. In essence, a role doesn't know its own type yet, because it
will be composed into another type. So if you happen to make any mention
of its main type (available as ::_), that mention is in
fact generic. Therefore the type of $self is generic. Likewise
if you refer to SUPER, the role doesn't know what the parent
classes are yet, so that's also generic. The actual types are instantiated
from the generic types when the role is composed into the class. (You
can use the role name ("Pet") directly, but only in places
where a role name is allowed as a type constraint, not in places that
declare the type of an actual object.)

Just as the body of a class declaration is actually a method call on
an instance of the MetaClass class, so too the body of
a role declaration is actually a method call on an instance of the MetaRole
class, which is like the MetaClass class, with some tweaks
to manage Role objects instead of Class objects.
For instance, a Role object doesn't actually support a
dispatcher like a Class object.

MetaRole and MetaClass do not inherit from
each other. More likely they both inherit from MetaModule
or some such.

Unlike certain other languages you may be altogether too familiar with,
Perl uses square brackets for parametric types rather than angles. Within
those square brackets it uses standard signature notation, so you can
also use the arguments to pass initial values, for instance. Just bear
in mind that by default any parameters to a role or class are considered
part of the name of the class when instantiated. Inasmuch as instantiated
type names are reminiscent of multimethod "long names", you may use
a colon to separate those arguments that are to be considered part of
the name from those that are just options.

Please note that these types can be as latent (or as non-latent) as you
like. Remember that what looks like compile time to you is actually
runtime to the compiler, so it's free to bind types as early or late
as you tell it to, including not at all.

When such a role is included in a class, the methods then have to be
defined by the class that uses the role. Actually, each method is on
its own--a role is free to define default implementations for any subset
of the methods it declares.

If a role declares private accessors, those accessors are private to
the class, not the role. The class must define any private implementations
that are not supplied by the role, just as with public methods. But
private method names are never visible outside the class (except to
its trusted proxy classes).

Unlike in the Traits paper, we allow roles to have state. Which is fancy
way of saying that the role can define attributes, and methods that
act on those attributes, not just methods that act only on other methods.

By the way, I think that when $.collar is undefined, calling
.tag on it should merely return undef rather
than throwing an exception (in the same way that @foo[$x][$y][$z]
returns undef when @foo[$x] is undefined,
and for the same reason). The undef object returned should,
of course, contain an unthrown exception documenting the problem, so
that if the undef is ever asked to provide a defined value,
it can explain why it can't do so. Or if the returned value is tested
by //, it can participate in the resulting error message.

If you want to parameterize the initial value of a role attribute, be
sure to put a colon if you don't want the parameter to be considered
part of the long name:

It gets worse. Since you can specify inheritance with an "is" declaration
within a class, you can do the same with a role:

role Pet {
is Friend;
}

Note carefully that this is not claiming that a Pet ISA
Friend (though that might be true enough). Roles never
inherit. So this is only saying that whatever animal takes on the role
of Pet gets some methods from Friend that
just happen to be implemented by inheritance rather than by composition.
Probably Friendshould have been written as a
role, but it wasn't (perhaps because it was written in Some Other Language
that runs on Parrot), and now you want to pretend that it was
written as a role to get your project out the door. You don't want to
use delegation because there's only one animal involved, and inheritance
will work good enough till you can rewrite Friend in a
language that supports role playing.

Of course, the really funny thing is that if you go across a language
barrier like that, Perl might just decide to emulate the inheritance
with delegation anyway. But that should be transparent to you. And if
two languages manage to unify their object models within the Parrot
engine, you don't want to suddenly have to rewrite your roles and classes.

And the really, really funny thing is that Parrot implements roles internally
with a funny form of multiple inheritance anyway...

Roles are most useful at compile time, or more precisely, at class composition
time, the moment in which the MetaClass class is figuring
out how to put together your Class object. Essentially,
that's while the closure associated with your class is being executed,
with a little extra happening before and after.

A class incorporates a role with the verb "does", like this:

class Dog is Mammal does Pet does Sentry {...}

or equivalently, within the body of the class closure:

class Dog {
is Mammal;
does Pet;
does Sentry;
...
}

There is no ordering dependency among the roles, so it doesn't matter
above if Sentry comes before Pet. That is
because the class just remembers all the roles and then meshes them
after the closure is done executing.

Each role's methods are incorporated into the class unless there is already
a method of that name defined in the class itself. A class's method
definition hides any role definition of the same name, so role methods
are second-class citizens. On the other hand, role methods are still
part of the class itself, so they hide any methods inherited from other
classes, which makes ordinary inherited methods third-class citizens,
as it were.

If there are no method name conflicts between roles (or with the class),
then each role's methods can be installed in the class, and we're done.
(Unless we wish to do further analysis of role interrelationships to
make sure that each role can find the methods it depends on, in which
case we can do that. But for 6.0.0 I'll be happy if non-existent methods
just fail at runtime as they do now in Perl 5.)

If, however, two roles try to introduce a method of the same name (for
some definition of name), then the composition of the class fails,
and the compilation of the program blows sky high--we sincerely hope.
It's much better to catch this kind of error at compile time if you
can. And in this case, you can.

There are several ways to solve conflicts. The first is simply to write
a class method that overrides the conflicting role methods, perhaps
figuring out which role method to call. It is allowed to use the role
name to select one of the hidden role methods:

So even though the methods were not officially composed into the class,
they're still there--they're not thrown away.

That last example looks an awful lot like multiple dispatch, and in fact,
if you declare the roles' methods with multi, they would
be treated as methods with different "long names", provided their signatures
were sufficiently different.

An interesting question, though, is whether the class can force two role
methods that weren't declared "multi" to behave as if they were. Perhaps
this can be forced if the class declares a signatureless multi stub
without defining it later in the class:

multi shake {...}

The Traits paper recommends providing ways of renaming or excluding one
or the other of the conflicting methods. We don't recommend that, because
it's better if you can keep both contracts through multiple dispatch
to the role methods. However, you can force renaming or exclusion by
pretending the role is a delegation:

While roles are at their most powerful at compile time, they can also
function as mixin classes at runtime. The "does" binary operator performs
the feat of deriving a new class and binding the object to it:

$fido does Sentry

Actually, it only does this if $fido doesn't already do
the Sentry role. If it does already, this is basically
a no-op. The does operator works on the object in place.
It would be illegal to say, for instance,

0 does true

The does operator returns the object so you can nest mixins:

$fido does Sentry does Tricks does TailChasing does Scratch;

Unlike the compile-time role composition, each of these layers on a new
mixin with a new level of inheritance, creating a new anonymous class
for dear old Fido, so that a .chase method from TailChasing
hides a .chase method from Sentry.

(Do not confuse the binary does with the unary does
that you use inside a class definition to pull in a role.)

In contrast to does, the but operator works
on a copy. So you can say:

0 but true

and you get a mixin based on a copy of 0, not the original 0, which everyone
shares. One other wrinkle is that "true" isn't, in fact, a class name.
It's an enumerated value of a bit class. So what we said was a shorthand
for something like:

0 but bit::true

In earlier Apocalypses we talked about applying properties with but.
This has now been unified with mixins, so any time you say:

This means that when you mention "prop" in your program,
something has to know how to map that to the SomeRole role.
That would often be something like an enum declaration. It's illegal
to use an undeclared property. But sometimes you just want a random
old property for which the role has the same name as the property. You
can declare one with

my property answer;

and that essentially declares a role that looks something like

my role answer {
has $.answer is rw = 1;
}

Then you can say

$a = 0 but answer(42)

and you have an object of an anonymous type that "does" answer,
and that include a .answer accessor of the same name, so
that if you call $a.answer, you'll get back 42.
But $a itself has the value 0. Since the accessor
is "rw", you can also say

$a.answer = 43;

There's a corresponding assignment operator:

$a but= tainted;

That avoids copying $a before tainting it. It basically
means the same thing as

Traits can be thought of as roles gone wrong. Like roles, they can function
as straightforward mixins on container objects at compile time, but
they can also cheat, and frequently do. Unlike roles, traits are not
constrained to play fair with each other. With traits, it's both "first
come, first served", and "he who laughs last laughs best". Traits are
applied one at a time to their container victim, er, object, and an
earlier trait can throw away information required by a later trait.
Contrariwise, a later trait can overrule anything done by an earlier
trait--except of course that it can't undestroy information that has
been totally forgotten by the earlier trait.

You might say that "role" is short for "role model", while "trait" is
short for "traitor". In a nutshell, roles are symbiotes, while traits
are parasites. Nevertheless, some parasites are symbiotic, and some
symbiotes are parasitic. Go figure...

All that being said, well-behaved traits are really just roles applied
to declared items like containers or classes. It's the declaration of
the item itself that makes traits seem more permanent than ordinary
properties. The only reason we call them "traits" rather than "properties"
is to continually remind people that they are, in fact, applied at compile
time. (Well, and so that we can make bad puns on "traitor".)

Even ill-behaved traits should add an appropriately named role to the
container, however, in case someone wants to look at the metadata properties
of the container.

Traits are generally inflicted upon the "traitee" with the "is" keyword,
though other modalities are possible. When the compiler sees words like
"is" or "will" or "returns" or "handles", or special constructs like
signatures and body closures, it calls into an associated trait handler,
which applies the role to the item as a mixin, and also does any other
traitorous magic that needs doing.

To define a trait handler for an "is xxx" trait, define one or more multisubs
into a property role like this:

which sets the "tie" type of the container to the implementation type
in $tied.

In any event, if the trait supplies the optional argument, that comes
in as $arg. (It's probably something unimportant, like
the function body...) Note that unlike "pair options" such as ":wag",
traits do not necessarily default to the value 1 if you don't supply
the argument. This is consistent with the notion that traits don't generally
do something passive like setting a value somewhere, but something active
like totally screwing up the structure of your container.

Most traits are introduced by use of a "helping verb", which could be
something like "is", or "will", or "can",
or "might", or "should", or "does".
We call these helping verbs "trait auxiliaries". Here's "will",
which (being syntactic sugar) merely delegates to back to "is":

Some traits are control freaks, so they want to make sure that anything
mentioning them comes through their control. They don't want something
dispatching to another trait's trait_help:is method just because someone
introduced a cute new container type they don't know about. That other
trait would just mess things up.

Of course, if a trait is feeling magnanimous, it should just go ahead
and use multi subs. Since the multi-dispatcher takes into account single-dispatch
methods, and the distance of an exact match on the first argument is
0, the dispatcher will generally respect the wishes of both the paranoid
and the carefree.

Note that we included "does" in our list of "helping verbs". Roles actually
implement themselves using the trait interface, but the generic version
of trait_auxiliary:does defaults to doing proper roley
things rather than proper classy things or improper traitorous things.
So yes, you could define your own trait_auxiliary:does
and turn your nice role traitorous. That would be...naughty.

But apart from how you typically invoke them, traits and roles are really
the same thing. Just like the roles on which they're based, you may
neither instantiate nor inherit from a trait. You may, however, use
their names as type constraints on multimethod signatures and such.
As with well-behaved roles, they should define attributes or methods
that show up as metadata properties where that's appropriate. Unlike
compile-time roles, which all flatten out in the same class, compile-time
traits are applied one at a time, like mixin roles. You can, in fact,
apply a trait to a container at runtime, but if you do, it's just an
ordinary mixin role. You have to call the appropriate trait_auxiliary:is()
routine yourself if you want it to do any extra shenanigans. The compiler
won't call it for you at runtime like it would at compile time.

When you define a helping verb such as "is" or "does", it not only makes
it a postfix operator for declarations, but a unary operator within
class and role closures. Likewise, declarative closure blocks like BEGIN
and INIT are actually trait verbs, albeit ones that can
add multiple closures to a queue rather than adding a single property.
This implies that something like

sub foo {
LEAVE {...}
...
}

could (except for scoping issues) equivalently be written:

sub foo LEAVE {...} {
...
}

Though why you'd want to that, I don't know. Hmm, if we really generalize
trait verbs like that, then you could also write things like:

sub foo {
is signature ('int $x');
is cached;
returns Int;
...
}

That's getting a little out there. Maybe we won't generalize it quite
that far...

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

Delegation is the art of letting someone else do your work for you. The
fact that you consider it "your" work implies that delegation is actually
a means of taking credit in advance for what someone else is going to
do. In terms of objects, it means pretending that some other object's
methods are your own. Now, as it happens, you can always do that by
hand simply by writing your own methods that call out to another object's
methods of the same name. So any shorthand for doing that is pure syntactic
sugar. That's what we're talking about here.

Delegation in this sugary sense always requires there to be an attribute
to keep a reference to the object we're delegating to. So our syntactic
relief will come in the form of annotations on a "has"
declaration. We could have decided to instead attach annotations to
each method declaration associated with the attribute, but by the time
you do this, you've repeated so much information that you almost might
as well have written the non-sugary version yourself. I know that for
a fact, because that's how I originally proposed it. :-)

Delegation is specified by a "handles" trait verb with an argument specifying
one or more method names that the current object and the delegated object
will have in common:

has $:tail handles 'wag';

Since the method name (but nothing else) is known at class construction
time, the following .wag method is autogenerated for you:

method wag (*@args is context(Lazy)) { $:tail.wag(*@args) }

(It's necessary to specify a Lazy context for the arguments
to a such a delegator method because the actual signature is supplied
by the tail's .wag method, not your method.) So as you
can see, the delegation syntax already cuts our typing in half, not
to mention the reading. The win is even greater when you specify multiple
methods to delegate:

Of course, it's illegal to call the outer method unless the attribute
has been initialized to an object of a type supporting the method. So
a declaration that makes a new delegatee at object build time might
be specified like this:

has $:tail handles 'wag' will build { Tail.new(*%_) };

or, equivalently,

has $:tail handles 'wag' = { Tail.new(*%_) };

This automatically performs

$:tail = Tail.new(*%_);

when BUILD is called on a new object of the current class
(unless BUILD initializes $:tail to some other
value). Or, since you might want to declare the type of the attribute
without duplicating it in the default value, you can also say

has Tail $:tail handles 'wag' = { .new(*%_) };

or

has Tail $:tail handles 'wag' will build { .new(*%_) };

Note that putting a Tail type on the attribute does not
necessarily mean that the method is always delegated to the Tail
class. The dispatch is still based on the runtime type of
the object, not the declared type. So

has Tail $:tail handles 'wag' = { LongTail.new(*%_) };

delegates to the LongTail class, not the Tail
class. Of course, you'll get an exception at build time if you try to
say:

has Tail $:tail handles 'wag' = { Dog.new(*%_) };

since Dog is not derived from Tail (whether
or not the tail can wag the dog).

We declare $:tail as a private attribute here, but $.tail
would have worked just as well. A Dog's tail does seem
to be a public interface, after all. Kind of a read-only accessor.

We've seen that the argument to "handles" can be a string
or a list of strings. But any argument or subargument that is not a
string is considered to be a smartmatch selector for methods. So you
can say:

has $:fur handles /^get_/;

and then you can do the .get_wet or .get_fleas
methods (presuming there are such), but you can't call the .shake
or .roll_in_the_dirt methods. (Obviously you don't want
to delegate the .shake method since that means something
else when applied to the Dog as a whole.)

If you say

has $:fur handles Groomable;

then you get only those methods available via the Groomable
role or class.

Wildcard matches are evaluated only after it has been determined that
there's no exact match to the method name. They therefore function as
a kind of autoloading in the overall pecking order. If the class also
has an AUTOLOAD, it is called only if none of the wildcard
delegations match. (An AUTOMETHDEF is called much earlier,
since it knows from the stub declarations whether there is supposed
to be a method of that name. So you can think of explicit delegation
as a kind of autodefine, and wildcard delegation as a kind of autoload.)

When you have multiple wildcard delegations to different objects, it's
possible to have a conflict of method names. Wildcard method matches
are evaluated in order, so the earliest one wins. (Non-wildcard method
conflicts can be caught at class composition time.)

If, where you would ordinarily specify a string, you put a pair, then
the pair maps the method name in this class to the method name in the
other class. If you put a hash, each key/value pair is treated as such
a mapping. Such mappings are not considered wildcards.

has $:fur handles { :shakefur«shake» :scratch«get_fleas» };

Perhaps that reads better with the old pair notation:

has $:fur handles { shakefur => 'shake', scratch => 'get_fleas' };

You can do a wildcard renaming, but not with pairs. Instead
do smartmatch with a substitution:

has $:fur handles (s/^furget_/get_/);

As always, the left-to-right mapping is from this class to the other
one. The pattern matching is working on the method name passed to us,
and the substituted method name is used on the class we delegate to.

Ordinarily delegation is based on an attribute holding an object reference,
but there's no reason in principle why you have to use an attribute.
Suppose you had a Dog with two tails. You can delegate
based on a method call:

method select_tail handles «wag hang» {...}

The arguments are sent to both the delegator and delegatee method. So
when you call

$dog.wag(:fast)

you're actually calling

$dog.select_tail(:fast).wag(:fast)

If you use a wildcard delegation based on a method, you should be aware
that it has to call the method before it can even decide whether there's
a valid method call to the delegatee or not. So it behooves you not
to get too fancy with select_tail(), since it might just
have to throw all that work away and go on to the next wildcard specification.

then something cool happens. <cool rays> In this case Perl 6 assumes
that your array contains a list of potential handlers, and you just
want to call the first one that succeeds. This is not considered
a wildcard match unless the "handles" argument forces it to be.

Note that this is different from the semantics of a hyper method such
as @objects».foo(), which will try to call the method
on every object in @objects. If you want to do
that, you'll just have to write your own method:

If the string is not found in the hash, a "next METHOD"
is automatically performed.

Again, this construct is not necessarily considered a wildcard. In the
example above we know for a fact that there's supposed to be a .bark
method somewhere, therefore a specific method can be autogenerated in
the current class.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

Delegation is a means of including a set of methods into your class.
Roles can also include a set of methods in your class, but the difference
is that what a role includes happens at class composition time, while
delegation is much more dynamic, depending on the current state of
the delegating attribute (or method).

But there's no reason you can't have your cake and eat it too, because
roles are specifically designed to allow you to pull in delegations
without the class even being aware of the fact that it's delegating.
When you include a role, you're just signing up for a set of methods,
with maybe a little state thrown in. You don't care whether those methods
are defined directly, or indirectly. The role manages that.

In fact, this is one of the primary motivators for including roles in
the design of Perl 6. As a named abstraction, a role lets you refactor
all the classes using that role without changing any of the classes
involved. You can turn your single "god" object into a set of nicely
cooperating objects transparently. Well, you have to do the composition
using roles first, and that's not transparent.

Note that all statically named methods are dispatched before any wildcard
methods, regardless of whether the methods came from a role or the class
itself. (Inherited methods also come before wildcard methods because
we order all the cachable method dispatches before all the non-cachable
ones. But see below.) So the lookup order is:

This class's declared methods (including autodefs and delegations)

An included role's declared methods (including autodefs and delegations)

Wildcard delegated methods in this class (or failing that, from
any inherited class that does wildcard delegations)

Methods autoloaded by an autoloader defined in this class (or failing
that, an autoloader from any inherited class)

Note that any method that is stubbed (declared but not yet defined) in
steps 1 or 2 skips straight to step 4, because it means this class thinks
it "owns" a method of that name. (At this point Perl 5 would skip straight
to step 5, but Perl 6 still wants to do wildcard delegation before falling
back on inherited autoloading.)

When you inherit from a class with a different layout policy, Perl has
to emulate inheritance via anonymous delegation. In this case it installs
a wildcard delegation for you. According to the list above, this gives
precedence to all methods with the same layout policy over all methods
with a different layout policy. This might be a feature, especially
when calling cross-language. Then again, maybe it isn't.

There is no "has" variable for such an anonymous delegation.
Its delegated object is stored as a property on the class's entry in
the ISA list, probably. (Or we could autogenerate an attribute whose
name is related to the class name, I suppose.)

Since one of the primary motivations for allowing this is to make it
possible to call back and forth between Perl 5 and Perl 6 objects, we
need to make that as transparent as possible. When a Perl 6 object inherits
from a Perl 5 object, it is emulated with delegation. The invocant passed
into the Perl 5 (Ponie) object looks like a Perl 5 object to Perl 5.
However, if the Perl 5 object passes that as an invocant back into Perl
6, it has to go back to looking like a Perl 6 object to Perl 6, or our
emulation of inheritance is suboptimal. When a Ponie object accesses
its attributes through what it thinks is a hash reference,
it really has to call the appropriate Perl 6 accessor function if the
object comes from Perl 6. Likewise, when Perl 6 calls an accessor on
a Perl 5 object, it has to translate that method call into a hash lookup--presuming
that the Perl 5 object is implemented as a blessed hash.

Other language boundaries may or may not do similar tricks. Python's
attributes suffer from the same misdesign as Perl 5's attributes. (My
fault for copying Python's object model. :-) So that'd
be a good place for a similar policy.

So we can almost certainly emulate inheritance with delegation, albeit
with some possible misordering of classes if there are duplicate method
names. However, the hard part is constructing objects. Perl 5 doesn't
enforce a policy of named arguments for its constructors, so it is difficult
for a Perl 6 BUILDALL routine to have any automatic way
to call a Perl 5 constructor. It's tempting to install glue code into
the Perl 6 class that will do the translation, but that's really not
a good idea, because someday the Perl 5 class may eventually get translated
to a Perl 6 class, and your glue code will be useless, or worse.

So the right place to put the glue is actually back into the Perl 5 class.
If a Perl 5 class defines a BUILD subroutine, it will be
assumed that it properly handles named pairs in Perl 5's even/odd list
format. That will be used in lieu of any predefined constructor named
"new" or anything else.

If there is no BUILD routine in the Perl 5 package, but
there is a "use fields" declaration, then we can autogenerate
a rudimentary BUILD routine that should suffice for most
scalar attributes.

I've always really liked the Ada distinction between types and subtypes.
A type is something that adds capabilities, while a subtype is something
that takes away capabilities. Classes and roles generally function as
types in Perl 6. In general you don't want to make a subclass that,
say, restricts your integers to only even numbers, because then you've
violated Liskov substitutability. In the same way that we force role
composition to be "before" classes, we will force subtyping constraints
to be "after" classes. In both cases we force it by a declarator change
so that you are unlikely to confuse a role with a class, or a class
with a subtype. And just as you aren't allowed to derive a role from
a class, you aren't allowed to derive a class from a constrained type.

On the other hand, a bit confusingly, it looks like subtyping will be
done with the "type" keyword, since we aren't using that word yet.

To remind people that a subtype of a class is just a constrained alias
for the class, we avoid the "is" word and declare a type using a ::=
compile-time alias, like this:

type Str_not2b ::= Str where /^[isnt|arent|amnot|aint]$/;

The ::= doesn't create the type, nor in fact does the type
keyword. It's actually the where that creates the type.
The type keyword just marks the name as "not really a classname"
so that you don't accidentally try to derive from it.

Since a type is "post-class-ical", there's really no such thing as an
object blessed into a type. If you try it, you'll just end up with an
object blessed into whatever the underlying unconstrained class is,
as far as inheritance is concerned. A type is not a subclass. A type
is primarily a handy way of sneaking smartmatching into multiple dispatch.
Just as a role allows you to specify something more general than a class,
a type allows you to specify something more specific than a class.

While types are primarily intended for restricting parameter types for
multiple dispatch, they also let you impose preconditions on assignment.
Basically, if you declare any container with a subtype, Perl will check
the constraint against any value you might try to bind or assign to
the container.

Given a set of multimethods that would "tie" on the actual classes of
the arguments, a multimethod with a matching constraint will be preferred
over an equivalent one with no constraint. So the first mesg
above is preferred if the constraint matches, and otherwise the second
is preferred. However, if two multis with constraints match (and are
otherwise equivalent), it's just as if you'd called any other set of
ambiguous multimethods, and one of them had better be marked as the
default, or you die.

We say that types are "post-class-ical", but since you can base them
off of any class including Any, they are actually rather
orthogonal to the class system.

You can import individual enums into your scope where they will function
like argumentless constant subs. However, if there is a name collision
with a sub or other enum, you'll have to disambiguate. Unambiguous enums
may be used as a property on the right side of a "but", and the enum
type can be intuited from it to make sure the object in question has
the right semantics mixed in. Two builtin enums are:

Likewise, a "final" class (to use the Java term) is one that you know
will never be derived from, let alone mucked with internally.

Now, it so happens that leaving all your classes open is not terribly
conducive to certain kinds of optimization (let alone encapsulation).
From the standpoint of the compiler, you'd like to be able to say, "I
know this class will never be derived from or modified, so I can do
things like access my attributes directly without going through virtual
accessors." We were, in fact, tempted to make closed classes the default.
But this breaks in frameworks like mod_perl where you cannot predict
in advance which classes will want to be extended or derived from.

Some languages solve this (or think they solve it) by letting classes
declare themselves to be closed and/or final. But that's actually a
bad violation of OO principles. It should be the users of a
class that decide such things--and decide it for themselves, not for
others. As such, there has to be a consensus among all users of a class
to close or finalize it. And as we all know, consensus is difficult
to achieve.

Nevertheless, the Perl 6 approach is to give the top-level application
the right to close (and finalize) classes. But we don't do this by simply
listing the classes we want to close. Instead, we use the sneaky strategy
of switching the default to closed and then list the classes we want
to stay open.

The benefit of this is that modules other than the top level can simply
list all the classes that they know should stay open. In an open framework,
these are, at worst, no-ops, and they don't cause classes to close that
other modules might want to remain open. If any module requests
a class to stay open, it stays open. If any module requests
that a class remain available as a base class, it remains available.

It has been speculated that optimizer technology in Parrot will develop
such that a class can conjecturally be compiled as closed, and then
recompiled as open should the need arise. (This is just a specific case
of the more general problem of what you do whenever the assumptions
of the optimizer are violated.) If we get such an on-the-fly optimizer/pessimizer,
then our open class declarations are still not wasted--they will tell
the optimizer which classes not to bother trying to close or finalize
in the first place. Setting the default the other way wouldn't have
the same benefit.

Syntax? You want syntax? Hmm.

use classes :closed :open«Mammal Insect»;

Or some such. Maybe certain kinds of class reference automatically request
the class to be open without a special pragma. A module could request
open classes without attempting to close everything with just:

use classes :open«Mammal Insect»;

On the other hand, maybe that's another one of those inside-out interfaces,
and it should just be options on the classes whose declarations you
have to include anyway:

use classes :closed;
class Mammal is open {...}
class Insect is open {...}

Similarly, we can finalize classes by default and then "take it back"
for certain classes:

In any event, even though the default is expressed at the top of the
main application, the final decision on each class is not made by the
compiler until CHECK time, when all the compiled code has
had a chance to stake its claims. (A JIT compiler might well wait even
longer, in case runtime evaluated code wishes to express an opinion.)

In theory, a subclass should always act as a more specialized version
of a superclass. In terms of the design-by-contract theory, a subclass should
OR in its preconditions and AND in its postconditions. In terms of Liskov
substitutability, you should always be able to substitute a derived
class object in where a base class object is expected, and not have
it blow up. In terms of Internet policy, a derived class (compared to
its base class) should be at least as lenient in what it accepts, and
at least as strict in what it emits.

So, while it would be lovely in a way to require that derived methods
of the same name as a base method must use the same signature, in practice
that doesn't work out. A derived class often has to be able to add arguments
to the signature of a method so that it can "be more lenient" in what
it accepts as input.

But this poses a problem, insofar as the user of the derived object does
not know whether all the methods of a given name support the same interface.
Under SUPER semantics, one can at least assume that the
derived class will "weed out" any arguments that would be detrimental
to its superclass. But as we have already pointed out, there isn't a
single superclass under MI, and each superclass might need to have different
"detrimental arguments" weeded out. One could say that in that case,
you don't call SUPER but rather call out to each superclass
explicitly. But then you're back to the problem that SUPER
was designed to solve. And you haven't solved SUPER's problem
either.

Under NEXT semantics, we assume that we are dispatching
to a set of methods with the same name, but potentially different signatures.
(Perl 6's SUPER implementation is really a limited form
of NEXT, insofar as SUPER indicates a set
of parent methods, unlike in Perl 5 where it picks one.) We need a way
of satisfying different signatures with the same set of arguments.

There are, in fact, two ways to approach this. One way is to say, okay
everything is a multimethod, and we just won't call anything whose signature
is irreconcilably inconsistent with the arguments presented. Plus there
are varying degrees of consistency within the set of "consistent" interfaces,
so we try them in decreasing order of consistency. A more consistent
multi is allowed to fall back to a less consistent multi with "next
METHOD".

But as a variant of the "pick one" mentality, that still doesn't help
the situation where you want to send a message to all your ancestor
classes (like "Please Mr. Base Class, help me initialize this object."),
but you want to be more specific with some classes than others ("Please
Miss Derived Class, set your $.prim attribute to 1.").
So the other approach is to use named arguments that can be ignored
by any classes that don't grok the argument.

So what this essentially comes down to is the fact that all methods and
submethods of classes that might be derived from (which is essentially
all classes, but see the previous section) must have a *%
parameter, either explicitly or implicitly, to collect up and render
harmless any unrecognized option pairs in the argument list. So the
ruling is that all methods and submethods that do not declare an explicit
*% parameter will get an implicit *%_ parameter
declared for them whether they like it or not. (Subroutines are not
granted this "favor".)

It might be objected that this will slow down the parameter binding algorithm
for all methods favored with an implicit *%_, but I would
argue that the binding code doesn't have to do anything till it sees
a named parameter it doesn't recognize, and then it can figure out whether
the method even references %_, and if not, simply throw
the unrecognized argument away instead of constructing a %_
that won't be used. And most of this "figuring out" can be done at compile
time.

Another counterargument is that this prevents a class from recognizing
typos in argument names. That's true. It might be possible to ask for
a warning that checks globally (at class-finalization time in the optimizer?)
to see if there is any method of that name anywhere that is interested
in a parameter of that name. But any class that gets its parameters
out of a *% hash at runtime would cause false positives,
unless we assume that any *% hash makes any argument name
legal, in which case we're pretty much back to where we started, unless
we do analysis of the usage of all *% hash in those methods,
and count things like %_«prim» as proper parameter
declarations. And that can still be spoofed in any number of ways. Plus
it's not a trivial warning to calculate, so it probably wouldn't be
the default in a load-and-go interpreter.

So I think we basically have to live with possible typos to get proper
polymorphic dispatch. If something is frequently misspelled, then you
could always put in an explicit test against %_ for that
argument:

warn "Didn't you mean :the(%_«teh»)?" if %_«teh»;

And perhaps we could have a pragma:

use signatures :exact;

But it's possible that the correct solution is to differentiate two kinds
of "isa", one that derives from "nextish" classes, and one that derives
from "superish" classes. A "next METHOD" traversal would
assume that any delegation to a super class would be handled explicitly
by the current class's methods. That is, a "superish" inheritance hides
the base class from .* and .+, as well as
"next METHOD".

On the other hand, if we marked the super class itself, we could refrain
from generating *% parameters for its methods. Any "next"
dispatcher would then have to "look ahead" to see if the next class
was a "superish" class, and bypass it. I haven't a clue what the syntax
should be though. We could mark the class with a "superish" trait, which
wouldn't be inheritable. Or we could mark it with a Superish role, which
would be inheritable, and a base class would have to override it to
impose a Nextish role instead. (But then what if one parent class is
Superish and one is Nextish?) Or we could even have two different metaclasses,
if we decide the two kinds of classes are fundamentally different beasts.
In that case we'd declare them differently using "class" and some other
keyword. Of course, people will want to use "class" for the type they
prefer, and the other keyword for the type they don't prefer. :-)

But since we're attempting to bias things in favor of nextish semantics,
that would be a "class", and the superish semantics might be a "guthlophikralique"
or some such. :-)

Seriously, if we mark the class, "is hidden" can hide the
current class from "next METHOD" semantics. The problem
with that is, how do you apply the trait to a class in a different language?
That argues for marking the "isa" instead. So as usual when we can't
make up our minds, we'll just have it both ways. To mark the class itself,
use "is hidden". To mark the "isa", use "hides Base"
instead of "is Base". In neither case will "next
METHOD" traverse to such a class. (And no *%_ will
be autogenerated.)

Since all the base classes are "next-aware", MyClass knows
it can just defer to "next" and both parent classes' dostuff
methods will be called. However, suppose one of our base classes is
old-fashioned and thinks it should call things with SUPER::
instead. (Or it's a class off in Python or Ruby.) Then we have to write
our classes more like this:

Here, MyClass knows that it has two very different base
classes. Nextish knows about "next", and Superish
doesn't. So it delegates to Superish::dostuff() differently
than it delegates to Nextish::dostuff(). The fact that
it declared "hides Superish" prevents next
from visiting the Superish class.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

We'd like to be able to support virtual inner classes. You can't have
virtual inner classes unless you have a way to dispatch to the actual
class of the invocant. That says to me that the solution is bound up
intimately with the method dispatcher, and the syntax of naming an inner
class has to know about the invocant in whose context we have to start
searching for the inner class. So we could have an explicit syntax like:

When you say Derived.new(), it creates a Derived
object, calls Derived::BUILDALL, which eventually calls
Base::BUILD, which makes a Middle::Inner object
(because that's what the virtual method $self.Inner returns)
and puts it in a variable that of the Base::Inner type
(which is fine, since Middle::Inner ISA Base::Inner.
Whew!

The only extra magic here is that an inner class would have to autogenerate
an accessor method (of the same name) that returns the class. A class
could then choose to access an inner class name directly, in which case
it would get its own inner class of that name, much like $.foo
always gets you your own attribute. But if you called the inner class
name as a method, it would automatically virtualize the name, and you'd
get the most derived existing version of the class.

This would give us most of what RFC 254 is asking for, at the expense
of one more autogenerated method. Use of such inner classes would take
the connivance of a base class that doesn't mind if derived classes
redefine its inner class. Unfortunately, it would have to express that
approval by calling $self.Inner explicitly. So this solution
does not go as far as letting you change classes that didn't expect
to be changed.

It would be possible to take it further, and I think we should. If we
say that whenever you use any global class, it makes an inner class
on your behalf that is merely an alias to the global class, creating
the accessor method as if it were an inner class, then it's possible
to virtualize the name of any class, as long as you're in a
context that has an appropriate invocant. Then we'd make any class name
lookup assume $self. on the front, basically.

This may seem like a wild idea, but interestingly, we're already proposing
to do a similar aliasing in order to have multiple versions of a module
running simultaneously. In the case of classes, it seems perfectly natural
that a new version might derive from an older version rather than redefining
everything.

The one fly in the ointment that I can see is that we might not always
have an appropriate invocant--for instance, outside any method body,
when we're declaring attributes. I guess when there's no dynamic context
indicating what an "inner" classname should mean, it should default
to the ordinary meaning in the current lexical and/or package context.
Within a class definition, for instance, the invocant is the metaclass,
which is unhelpful. So generally that means that a declared attribute
type will turn out to be a superclass of the actual attribute type at
runtime. But that's fine, ain't it? You can always store a Beagle
in a Dog attribute.

So in essence, it boils down to this. Within a method, the invocant is
allowed to have opinions about the meanings of any class names, and
when there are multiple possible meanings, pick the most appropriate
one, where that amounts to the name you'd find if the class name were
a virtual method name.

Here's the example from RFC 254, translated to Perl 6 (with Frog
made into an explicit inner class for clarity (though it should work
with any class by the aliasing rule above)):

In the Perl 5 equivalent, that would have printed "ribbit ribbit" instead.
How did it do the right thing in Perl 6?

The difference is on the line marked "MAGIC". Because Frog
was mentioned in a method, and the invocant was of type Forest::Japanese
rather than of type Forest, the word "Frog"
figured out that it was supposed to mean a Forest::Japanese::Frog
rather than a Forest::Frog. The name was "virtual". So
we ended up creating a forest with a frog of the appropriate type, even
though it might not have occurred to the writer of Forest
that a subclass would override the meaning of Frog.

So one object can think that its Frog is Japanese, while
another thinks it's Russian, or Mexican, or even Antarctican (if you
can find any forests there). Base methods that talk about Frog
will automatically find the Frog appropriate to the current
invocant. This works even if Frog is an outer class rather
than an inner class, because any outer class referenced by a base class
is automatically aliased into the class as a fake inner class. And the
derived class doesn't have to redefine its Frog by declaring
an inner class either. It can just alias (or use) a different outer
Frog class in as its fake inner class. Or even a different
version of the same Frog class, if there are multiple versions
of it in the library.

It's also possible to put a collection of classes into a module, but
that doesn't buy you much except the ability to pull them all in with
one use, and manage them all with one version number. Which
has a lot to be said for it--in the next section.

Way back at the beginning, we claimed that a file-scoped class declaration:

class Dog;
...

is equivalent to the corresponding block-scoped declaration:

class Dog {...}

While that's true, it isn't the whole truth. A file-scoped class (or
module, or package) is the carrier of more metadata than a block-scoped
declaration. Perl 6 supports a notion of versions that is file based.
But even a class name plus a version is not sufficient to name a module--there
also has to be a naming authority, which could be a URI or a CPAN id.
This will be discussed more fully in Apocalypse 11, but for now we can
make some predictions.

The extra metadata has to be associated with the file somehow. It may
be implicit in the filename, or in the directory path leading to the
file. If so, then Perl 6 has to collect up this information as modules
are loaded and associate it with the top level class or module as a
set of properties.

It's also possible that a module could declare properties explicitly
to define these and other bits of metadata:

Modules posted to CPAN or entered into any standard Perl 6 library are
required to declare some set of these properties so that installations
can know where to keep them, such that multiple versions by different
authors can coexist, all of them available to any installed version
of Perl. (This is a requirement for any Perl 6 installation. We're tired
of having to reinstall half of CPAN every time we patch Perl. We also
want to be able to run different versions of the Frog module
simultaneously when the Frog requirements of the modules
we use are contradictory.)

It's possible that the metadata is supplied by both the declarations
and by the file's name or location in the library, but if so, it's a
fatal error to use a module for which those two sources contradict each
other as to author or version. (In theory, it could also be a fatal
error to use modules with incompatible licensing, but a kind warning
might be more appreciated.) Likely there will also be some kind of automatic
checksumming going on as well to prevent fraudulent distributions of
code.

It might simplify things if we make an identifier metadatum
that incorporates all of naming authority, package name, and version.
But the individual parts still have to be accessible, if only as components
of identifier. However we structure it, we should make
the identifier the actual declared full name of the class,
yet another one of those "long names" that include extra parameters.

Or maybe using email addresses is a bad idea now in the modern Spam Age.
Or maybe Spam Ages should be plural, like the Dark Ages...

In any event, such a declaration automatically aliases the full name
of the class (or module) to the short name. So for the rest of the scope,
Dog refers to the longer name.

(Though if you refer to Dog within a method, it's considered
a virtual class name, so Perl will search any derived classes for a
redefined inner Dog class (or alias) before it settles
on the least-derived aliased Dog class.)

We lied slightly when we said earlier that only the file-scoped class
carries extra metadata. In fact, all of the classes (or modules, or
packages) defined within your file carry metadata, but it so happens
that the version and author of all your extra classes (or modules, or
packages) are forced to be the same as the file's version and author.
This happens automatically, and you may not override the generation
of these long names, because if you did, different file versions could
and would have version collisions of their interior components, and
that would be catastrophic. In general you can ignore this, however,
since the long names of your extra classes are always automatically
aliased back down to the short names you thought you gave them in the
first place. The extra bookkeeping is in there only so that Perl can
keep your classes straight when multiple versions are running at the
same time. Just don't be surprised when you ask for the name of the
class and it tells you more than you expected.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

Since these long names are the actual names of the classes, when you
say:

use Dog;

you're really asking for something like:

use Dog-(Any)-(Any);

And when you say:

use Dog-1.2.1;

you're really asking for:

use Dog-1.2.1-(Any);

Note that the 1.2.1 specifies an exact match on
the version number. You might think that it should specify a minimum
version. However, people who want stable software will specify an exact
version and stick with it. They don't want 1.2.1 to mean
a minimum version. They know 1.2.1 works, so they want
that version nailed down forever--at least for now.

It's easy to specify what Perl 6 will provide for introspection: the
union of what Perl 6 needs and whatever Parrot provides for other languages.
;-)

In the particular case of class metadata, the interface should generally
be via the class's metaclass instance--the object of type MetaClass
that was in charge of building the class in the first place. The metamethods
are in the metaobject, not in the class object. (Well, actually, those
are the same object, but a class object ignores the fact that it's also
a metaobject, and dispatches by default to its own methods, not the
ones defined by the metaclass.) To get to the metamethods of an ordinary
class object you have to use the .meta method:

Unless MyClass has defined or inherited a .getmethods
method, the first call is an error. The second is guaranteed to work
for Perl 6's standard MetaClass objects. You can also call
.meta on any ordinary object:

$obj.meta.getmethods();

That's equivalent to

$obj.dispatcher.meta.getmethods();

As for which parts of a class are considered metadata--they all are,
if you scratch hard enough. Everything that is not stored directly as
a trait or property really ought to have some kind of trait-like method
to access it. Even the method body closures have to be accessible as
traits, since the .wrap method needs to have something
to put its wrapper around.

Minimally, we'll have user-specified class traits that look like this:

The layout determines whether one class can actually derive
from another or has to fake it. Any P6opaque class can compatibly inherit
from any other P6opaque class, but if it inherits from any P5 class,
it must use some form of delegation to another invocant. (Hopefully
with a smart enough invocant reference that, if the delegated object
unknowingly calls back into our layout system, we can recover the original
object reference and maintain some kind of compositional integrity.)

The metaclass's .getmethods method returns method-descriptor
objects with at least the following properties:

name the name of the method
signature the parameters of the method
returns the return type of the method
multi whether duplicate names are allowed
do the method body

The .getmethods method has a selector parameter that lets
you specify whether you want to see a flattened or hierarchical view,
whether you're interested in private methods, and so forth. If you want
a hierarchical view, you only get the methods actually defined in the
class proper. To get at the others, you follow the "isa" trait to find
your parent classes' methods, and you follow the "roles" trait to get
to role methods, and from parents or roles you may also find links to
further parents or roles.

The .getattributes method returns a list of attribute descriptors
that have traits like these:

name
type
scope
rw
private
accessor
build

Additionally they can have any other variable traits that can reasonably
be applied to object attributes, such as constant.

Strictly speaking, metamethods like .isa(), .does(),
and .can() should be called through the meta object:

$obj.meta.can("bark")
$obj.meta.does(Dog)
$obj.meta.isa(Mammal)

And they can always be called that way. For convenience you can often
omit the .meta call because the base Object
type translates any unrecognized .foo() into .meta.foo()
if the meta class has a method of that name. But if a derived class
overrides such a metamethod, you have to go through the .meta
call explicitly to get the original call.

In previous Apocalypses we said that:

$obj ~~ Dog

calls:

$obj.isa(Dog)

That is not longer the case--you're actually calling:

$obj.meta.does(Dog)

which is true if $obj either "does" or "isa" Dog
(or "isa" something that "does" Dog). That is, it asks
if $obj is likely to satisfy the interface that comes from
the Dog role or class. The .isa method, by
contrast, is strictly asking if $obj inherits from the
Dog class. It's erroneous to call it on a role. Well, okay,
it's not strictly erroneous. It will just never return true. The optimizer
will love you, and remove half your code.

Note that either of .does or .isa can lie,
insofar as you might include an interface that you later override parts
of. When in doubt, rely on .can instead. Better yet, rely
on your dispatcher to pick the right method without trying to second
guess it. (And then be prepared to catch the exception if the dispatcher
throws up its hands in disgust...)

By the way, unlike in Perl 5 where .can returns a single
routine reference, Perl 6's version of .meta.can returns
a "WALK" iterator for a set of routines that match the name. When dereferenced,
the iterator gets fed to a dispatcher as if the method had been called
in the first place. Note that any wildcard methods (via delegation or
AUTOLOAD) are included by default in this list of potential
handlers, so there is no reason for subclasses to have to redefine .can
to reflect the new names. This does potentially weaken the meaning of
.can from "definitely has a method of this name" to "definitely
has one or more methods in one or more classes that will try to handle
this." But that's probably closer to what you want, and the best we
can do when people start fooling around with wildcard methods under
MI.

However, that being said, many classes may wish to dynamically specify
at the last moment which methods they can or cannot handle. That is,
they want a hook to allow a class to declare names even while the .can
candidate list is being built. By default .meta.can includes
all wildcard delegations and autoloads at the end of the list. However,
it will exclude from the list of candidates any class that defines its
own AUTOMETH method, on the assumption that each such AUTOMETH
method has already had its chance to add any callable names to the list.
If the class's AUTOMETH wishes to supply a method, it should
return a reference to that method.

Do not confuse AUTOMETH with AUTOMETHDEF. The
former is equivalent to declaring a stub declaration. The latter is
equivalent to supplying a body for an existing stub. Whether AUTOMETH
actually creates a stub, or AUTOMETHDEF actually creates
a body, is entirely up to those routines. If they wish to cache their
results, of course, then they should create the stub or body.

There are corresponding AUTOSUB and AUTOSUBDEF
hooks. And AUTOVAR and AUTOVARDEF hooks. These
all pretty much make AUTOLOAD obsolete. But AUTOLOAD
is still there for old times's sake.

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

A lot of time went by while I was in the hospital last year, so we ended
up polishing up the design of Perl 6 in a number of areas not directly
related to OO. Since I've already got your attention (and we're already
90% of the way through this Apocalypse), I might as well list these
decisions here.

Compared to Perl 5, we've basically made it easier to mark something
as exportable, but more difficult to export something by default. You
no longer have to declare your tagsets separately, since :foo
parameters are self-declaring, and the module will automatically build
the tagsets for you from the export trait arguments.

We used one example of the conjectural gather/take construct. A gather
executes a closure, returning a list of all the values returned by take
within its lexical scope. In a lazy context it might run as a coroutine.
There probably ought to be a dynamically scoped variant. Unless it should
be dynamic by default, in which case there probably ought to be a lexically
scoped variant...

There's a new pair syntax that is more conducive to use as option arguments.
This syntax is reminiscent of both the Unix command line syntax and
the I/O layers syntax of Perl 5. But unlike Unix command-line options,
we use colon to introduce the option rather than the overly negative
minus sign. And unlike Perl 5's layers options, you can use these outside
of a string.

We haven't discarded the old pair syntax. It's still more readable for
certain uses, and it allows the key to be a non-identifier. Plus we
can define the new syntax in terms of it:

It's that last one that's the real winner for passing boolean options.
One other nice thing is that if you have several options in a row you
don't have to put commas between:

$handle = open $file, :chomp :encoding«guess» :ungzip or die "Oops";

It might be argued that this conflicts the :foo notation for private
methods. I don't think it's a problem because method names never occur
in isolation.

Oh, one other feature of option pairs is that certain operations can
use them as adverbs. For instance, you often want to tell the range
operator how much to skip on each iteration. That looks like this:

1..100 :by(3)

Note that this only works where an operator is expected rather than a
term. So there's no confusion between:

randomlistop 1..100 :by(3)

and

randomlistop 1..100, :by(3)

In the latter case, the option is being passed to randomlistop()
rather than the infix:.. operator.

Novice Perl 5 programmers are continually getting trapped by subscripts
that autoquote unexpectedly. So in Perl 6, we'll remove that special
case. %hash{shift} now always calls the shift function, because the
inside of curlies is always an expression. Instead, if you want to subscript
a hash with a constant string, or a slice of constant strings, use the
new French qw//-ish brackets like this:

%hash«alice» # same as %hash{'alice'}
%hash«alice bob charlie» # same as %hash{'alice','bob','charlie'}

Note in particular that, since slices in Perl 6 are determined by the
subscript only, not the sigil, this:

%hash«alice» = @x;

evaluates the right side in scalar context, while

%hash«alice bob charlie» = @x;

evaluates the right side in list context. As with all other uses of the
French quotes in Perl 6, you can always use:

%hash<<alice>> = @x;

if you can't figure out how to type ^K<< or ^K>>
in vim.

On the other hand, if you've got a fully Unicode aware editor, you could
probably write some macros to use the big double angles from Asian languages:

%hash《alice》 = @x;

But by default we only provide the Latin-1 compatible versions. It would
be easy to overuse Unicode in Perl 6, so we're trying to underuse Unicode
for small values of 6. (Not to be confused with ⁶, or ⅵ.)

The mathematicians got confused when we started talking about "vector"
operators, so these dimensionally dwimming versions of scalar operators
are now called hyper operators (again). Some folks see operations like

@a »*« @b

as totally useless, and maybe they are--to a mathematician. But to someone
simply trying to calculate a bunch of things in parallel (think cellular
automata, or aerodynamic simulations, for instance), they make a lot
of sense. And don't restrict your thinking to math operators. How about
appending a newline to every string before printing it out:

print @strings »~« "\n";

Of course,

for @strings {say}

is a shorter way to do the same thing. ("say" is just Perl
6's version of a printline function.)

In Apocalypse 2 we said that any method interpolated into a double-quoted
string has to have parentheses. We're throwing out that special rule
in the interests of consistency. Now if you want to interpolate a variable
followed by an "accidental" dot, use one of these:

There is a new =:= identity operator, which tests to see
if two objects are the same object. The association with the :=
binding operator should be obvious. (Some classes such as integers may
consider all objects of the same value to be a single object, in a Platonic
sense.)

Hmm? No, there is no associated assignment operator. And if there were,
I wouldn't tell you about it. Sheesh, some people...

Now, you may be thinking that some of these have long, unwieldy names.
You'd be right. The longer the name, the longer you should think before
adding a new operator of that category. (And the length of time you
should think probably scales exponentially with the length of the name.)

Editor's Note: this Apocalypse is out of date and remains here for historic reasons. See Synopsis 12 for the latest information.

As we talked about earlier, assignment to a "has" variable
is really pseudo-assignment representing a call to the "build"
trait. In the same way, assignment to state variables (Perl's
version of lexically scoped "static" variables), is taken as pseudo-assignment
representing a call to the "first" trait. The first time
through a piece of code is when state variables typically like to be
initialized. So saying:

state $pc = $startpc;

is equivalent to

state $pc is first( $startpc );

which means that it will pay attention to the $startpc variable
only the first time this block is ever executed. Note that any side
effects within the expression will only happen the first time through.
If you say

state $x = $y++;

then that statement will only ever increment $y once. If
that's not what you want, then use a real assignment as a separate statement:

state $x;
$x = $y++;

The := and .= operators also attempt to do
what you mean, which in the case of:

state $x := $y++;

still probably doesn't do what you want. :-)

In general, any "preset" trait is smart about when to apply its value
to the container it's being applied to, such that the value is set statically
if that's possible, and if that's not possible, it is set dynamically
at the "correct" moment.

For ordinary assignment to a "my" variable, that correct
moment just happens to be every time it is executed, so =
represents ordinary assignment. If you want to force an initial value
at execution time that was calculated earlier, however, then just use
ordinary assignment to assign the results of a precalculated block:

my @canines = INIT { split slurp "%ENV«HOME»/.canines" };

It's only the has and state declarators that
redefine assignment to set defaults with traits. (For has,
that's because the actual attribute variable won't exist until the object
is created. For state, that's because we want the default
to be "first time through".) But you can use any of the traits on any
variable for which it makes sense. For instance, just because we invented
the "first" initializer for state variables:

state $lexstate is first(0);

doesn't mean you can't use it to initialize any variable only the first
time through a block of code:

my $foo is first(0);

However, it probably doesn't make a lot of sense on a "my"
variable, unless you really want it to be undefined the second time
through. It does make a little more sense on an "our" variable
that will hang onto its value like a state variable:

our $counter is first(0);

An assignment would often be wrong in this case. But generally, the naive
user can simply use assignment, and it will usually do what they want
(if occasionally more often than they want). But it does exactly what
they want on has and state variables--presuming
they are savvy enough to want what it actually does... :-)

So as with has variables, state variables can
be initialized with precomputed values:

You can also use %hash.elems, which returns the number of
pairs in the hash. I don't think %hash.chars is terribly
useful, but it will tell you how many characters total there are in
the values. (The key lengths are ignored, just like the integer "keys"
of an ordinary array.)

Actually, the meaning of .chars varies depending on your
current level of Unicode support. To be more specific, there's also:

$string.bytes
$string.codepoints
$string.graphemes
$string.letters

...none of which should be confused with:

$string.columns

or its evil twin:

$string.pixels

Those last two require knowledge of the current font and rendering engine,
in fact. Though .columns is likely to be pretty much the
same for most Unicode fonts that restrict themselves to single and double-wide
characters.

A corollary to the preceding is that string positions are not numbers.
If you say either

$pos = index($string, "foo");

or

$string ~~ /foo/; $pos = $string.pos;

then $pos points to that location in that string. If you
ask for the numeric value of $pos, you'll get a number,
but which number you get can vary depending on whether you're currently
treating characters as bytes, codepoints, graphemes, or letters. When
you pass a $pos to substr($string, $pos, 3),
you'll get back "foo", but not because it counted over
some number of characters. If you use $pos on some other
string, then it has to interpret the value numerically in the current
view of what "character" means. In a boolean context, a position is
true if the position is defined, even if that position would evaluate
to 0 numerically. (index and rindex return
undef when they "run out".)

And, in fact, when you say $len = .chars, you're really
getting back the position of the end of the string, which just happens
to numerify to the number of characters in the string in the current
view. A consequence of the preceding rules is that "".chars
is true, but +"".chars is false. So Perl 5 code
that says length($string) needs to be translated to +chars($string)
if used in a boolean context.

Routines like substr and index take either
positions or integers for arguments. Integers will automatically be
turned into positions in the current view. This may involve traversing
the string for variable-width representations, especially when working
with combining characters as parts of graphemes. Once you're working
with abstract positions, however, they are efficient. So

while $pos = index($string, "fido", $pos + 1) {...}

never has to rescan the string.

The other point of all this is that you can pass $pos or
$len to another module, and it doesn't matter
if you're doing offsets in graphemes and they are doing offsets in codepoints.
They get the correct position by their lights, even though the number
of characters looks different. The main constraint on this is that if
you pass a position from a lower Unicode support level to a higher Unicode
support level, you can end up with a position that is inside what you
think of as a unitary character, whether that's a byte within a codepoint,
or a codepoint within a grapheme or letter. If you deref such a position,
an exception is thrown. But generally high-level routines call into
low-level routines, so the issue shouldn't arise all that often in practice.
However, low-level routines that want to be called from high-level routines
should strive not to return positions inside high-level characters--the
fly in the ointment being that the low-level routine doesn't necessarily
know the Unicode level expected by the calling routine. But we have
a solution for that...

High-level routines that suspect they may have a "partial position" can
call $pos.snap (or $pos.=snap) to round up
to the next integral position in the current view, or (much less commonly)
$pos.snapback (or $pos.=snapback) to round
down to the next integral position in the current view. This only biases
the position rightward or leftward. It doesn't actually do any repositioning
unless we're about to throw an exception. So this allows the low-level
routine to return $pos.snap without knowing at the time
how far forward to snap. The actual snapping is done later when the
high-level routine tries to use the position, and at that point we know
which semantics to snap forward under.

By the way, if you bind to a position rather than assign, it tracks the
string in question:

Deletions of string around a position cause the position to be reduced
to the beginning of the deletion. Insertions at a position are assumed
to be after that position. That is, the position stays pointing to the
beginning of the newly inserted string, like this:

Analogous to the disjunctional | separator, we're also putting
in a conjunctional & separator into our regex syntax:

"DOG" ~~ /D [ <vowel>+ & <upper>+ ] G/

The semantics of it are pretty straightforward, as long as you realize
that all of the AND'ed assertions have to match with the same length.
More precisely, they have to start and stop matching at the same location.
So the following is always going to be false:

/ . & .. /

It would be possible to have the other semantics where, as long as the
trailing assertion matches either way, it doesn't have to match the
trailing assertion the same way. But then tell me whether $1
should return "O" or "G" after this:

"DOG" ~~ /^[. & ..] (.)/

Besides, it's easy enough to get the other semantics with lookahead assertions.
Autoanchoring all the legs of a conjunction to the same spot adds much
more value to it by differentiating it from lookahead. You have to work
pretty hard to make separate lookaheads match the same length. Plus
doing that turns what should be a symmetric operator into a non-symmetrical
one, where the final lookahead can't be a lookahead because someone
has to "eat" the characters that all the assertions have agreed on are
the right number to eat. So for all these reasons it's better to have
a conjunction operator with complicated enough start/stop semantics
to be useful.

Actually, this operator was originally suggested to me by a biologist.
Which leads us to our...